SOx CONCENTRATION ACQUIRING APPARATUS OF INTERNAL COMBUSTION ENGINE

ABSTRACT

A SOx concentration acquiring apparatus of an internal combustion engine of the invention acquires sensor currents as SOx concentration currents after a sensor voltage reaches an oxygen decreasing voltage in a reoxidation voltage decreasing control, acquires the sensor current as a base current when the sensor voltage is equal to or lower than the oxygen decreasing voltage in the reoxidation voltage decreasing control, acquires an integration value of differences between the base current and each of the SOx concentration currents, and acquires a SOx concentration of an exhaust gas discharged from an internal combustion engine on the basis of the integration value.

BACKGROUND Field

The invention relates to a SOx concentration acquiring apparatus of aninternal combustion engine.

Description of the Related Art

There is known a SOx concentration acquiring apparatus for acquiring aconcentration of sulfur oxide included in an exhaust gas discharged froman internal combustion engine (for example, see JP 2015-17931 A). Theknown SOx concentration acquiring apparatus (hereinafter, will bereferred to as “the known apparatus”) comprises a limiting currentsensor. The limiting current sensor includes solid electrolyte layers, afirst sensor electrode, and a second sensor electrode. The first andsecond sensor electrodes are provided such that one of the solidelectrolyte layers is positioned between the first and second sensorelectrodes. Further, the first sensor electrode is provided such thatthe first sensor electrode exposes to the exhaust gas discharged fromthe internal combustion engine. Hereinafter, the concentration of thesulfur oxide will be referred to as “the SOx concentration”, and theconcentration of the sulfur oxide included in the exhaust gas justdischarged from the internal combustion engine will be referred to as“the exhaust SOx concentration”.

The known apparatus increases a voltage applied to the second sensorelectrode so as to produce an electric potential difference with respectto the first sensor electrode and then, decreases the voltage.

The known apparatus acquires the exhaust SOx concentration on the basisof a current flowing between the first and second sensor electrodeswhile the known apparatus decreases the voltage. Hereinafter, thevoltage applied to the second sensor electrode will be referred to as“the sensor voltage”, and the current flowing between the first andsecond sensor electrodes will be referred to as “the sensor current”.

The known apparatus is configured to acquire the exhaust SOxconcentration on the basis of a knowledge that the sensor currentcorrelates with the exhaust SOx concentration while the sensor voltagedecreases after the sensor voltage increases. According to thisknowledge, the known apparatus acquires, as a SOx concentration current,the sensor current at a point of time that the sensor voltage reaches acertain voltage while the known apparatus decreases the sensor voltageand acquires the exhaust SOx concentration on the basis of the acquiredSOx concentration current.

In this regard, the exhaust SOx concentration can be acquired accuratelyby using a parameter which changes at a rate larger than a rate, atwhich the SOx concentration current changes when the exhaust SOxconcentration changes.

SUMMARY

The invention has been made for solving the above-mentioned problems. Anobject of the invention is to provide a SOx concentration acquiringapparatus of the internal combustion engine which can acquire theexhaust SOx concentration accurately on the basis of the parameter whichchanges at a large rate in response to a change of the exhaust SOxconcentration.

A SOx concentration acquiring apparatus of an internal combustion engine(50) according to the first invention comprises a sensor cell (15, 26)and an electronic control unit (90). The sensor cell (15, 26) is formedby a solid electrolyte layer (11, 21A), a first sensor electrode (15A,26A), and a second sensor electrode (15B, 26B). The first sensorelectrode (15A, 26A) is provided on one of opposite surfaces of thesolid electrolyte layer (11, 21A) such that the first sensor electrode(15A, 26A) exposes to an exhaust gas discharged from the internalcombustion engine (50). The second sensor electrode (15B, 26B) isprovided on the other surface of the solid electrolyte layer (11, 21A).

The electronic control unit (90) controls a sensor voltage (Vss) whichis a voltage applied to the sensor cell (15, 26) and acquires a sensorcurrent (Iss) which is a current flowing through the sensor cell (15,26).

The electronic control unit (90) is configured to execute adecomposition voltage increasing control for increasing the sensorvoltage (Vss) from a first voltage lower than an oxygen increasingvoltage (Vox_in) to a second voltage (Vup_end) equal to or higher thanthe oxygen increasing voltage (see a step 920 in FIG. 9 and a step 1710in FIG. 17). The oxygen increasing voltage (Vox_in) is a voltage, atwhich an amount of oxygen component produced by SOx decomposing tosulfur component and the oxygen component is larger than the amount ofthe oxygen component consumed by the sulfur component being oxidized bythe oxygen component to the SOx. The electronic control unit (90) isfurther configured to execute a reoxidation voltage decreasing controlfor decreasing the sensor voltage (Vss) from the second voltage(Vup_end) to a third voltage (Vdown_end) lower than an oxygen decreasingvoltage (Vox_de) after the electronic control unit (90) executes thedecomposition voltage increasing control (see a step 940 in FIG. 9 and astep 1730 in FIG. 17). The oxygen decreasing voltage (Vox_de) is avoltage, at which the amount of the oxygen component consumed by thesulfur component being oxidized by the oxygen component to the SOx islarger than the amount of the oxygen component produced by the SOxdecomposing to the sulfur component and the oxygen component. Theelectronic control unit (90) is further configured to acquire the sensorcurrents (Iss) as SOx concentration currents (Iss_sox), respectivelyafter the sensor voltage (Vss) reaches the oxygen decreasing voltage(Vox_de) while the electronic control unit (90) executes the reoxidationvoltage decreasing control (see a step 945 in FIG. 9). The electroniccontrol unit (90) is further configured to acquire the sensor current(Iss) as a base current Ibase) when the sensor voltage (Vss) is equal toor lower than the oxygen decreasing voltage (Vox_de) while theelectronic control unit (90) executes the reoxidation voltage decreasingcontrol (see a step 1010 in FIG. 10). The electronic control unit (90)is further configured to acquire an integration value (S11, S12) ofdifferences (dIss) between the base current (Ibase) and each of the SOxconcentration currents (Iss_sox) (see a step 1030 in FIG. 10). Theelectronic control unit (90) is further configured to acquire a SOxconcentration (Csox) of the exhaust gas on the basis of the integrationvalue (S11, S12) (see a step 1040 in FIG. 10).

The inventors of this application have a knowledge that the sensorcurrent acquired when the exhaust gas includes the SOx, is lower thanthe sensor current acquired when the exhaust gas includes no SOx afterthe sensor voltage reaches the oxygen decreasing voltage while thereoxidation voltage decreasing control is executed. Thus, the inventorshave a knowledge that there is a difference between the sensor currentacquired when the exhaust gas includes the SOx and the sensor currentacquired when the exhaust gas include no SOx after the sensor voltagereaches the oxygen decreasing voltage while the reoxidation voltagedecreasing control is executed. The inventors of this application haveunderstood reasons for the difference as follows.

When the exhaust gas includes the SOx, the SOx decomposes to the sulfurcomponent and the oxygen component at the first sensor electrode afterthe sensor voltage reaches the oxygen increasing voltage while thedecomposition voltage increasing control for increasing the sensorvoltage is executed. The sulfur component produced by the decompositionof the SOx adheres to the first sensor electrode.

After the sensor voltage reaches the oxygen decreasing voltage while thereoxidation voltage decreasing control for decreasing the sensorvoltage, the sulfur component adhering to the first sensor electrode isreoxidized by the oxygen component around the first sensor electrode,thereby returning to the SOx. While the sulfur component is oxidized tothe SOx, the SOx may decompose to the sulfur component and the oxygencomponent at the first sensor electrode. However, the oxidizing reactionof the sulfur component adhering to the first sensor electrode isdominant, compared with the decomposing reaction of the SOx. As aresult, the amount of the oxygen component consumed by the oxidation ofthe sulfur component is larger than the amount of the oxygen componentproduced by the decomposition of the SOx. Thus, an amount of oxygen ionflowing through the sensor cell decreases. As a result, the sensorcurrent decreases.

For the reasons, the inventors of this application have understood thatthe sensor current acquired when the exhaust gas includes the SOx islower than the sensor current acquired when the exhaust gas includes noSOx after the sensor voltage reaches the oxygen decreasing voltage whilethe reoxidation voltage decreasing control is executed.

The SOx concentration acquiring apparatus according to the firstinvention acquires the SOx concentration by using the SOx concentrationcurrents acquired after the sensor voltage reaches the oxygen decreasingvoltage. The SOx concentration currents are the sensor currents acquiredwhen the amount of the oxygen component consumed by the oxidation of thesulfur component to the SOx, is larger than the amount of the oxygencomponent produced by the decomposition of the SOx to the sulfurcomponent and the oxygen component. Therefore, the SOx concentrationcurrents are the sensor currents subject to the oxidizing reaction ofthe sulfur component. Further, the SOx concentration acquiring apparatusacquires the SOx concentration by using the base current acquired whenthe sensor voltage is equal to or lower than the oxygen decreasingvoltage. Therefore, the base current also corresponds to the sensorcurrent subject to the oxidizing reaction of the sulfur component.

Further, the SOx concentration acquiring apparatus according to thefirst invention uses the integration value for acquiring the SOxconcentration. The integration value value acquired by integrating thedifferences between the base current subject to the oxidizing reactionof the sulfur component and each of the SOx concentration currentssubject to the oxidizing reaction of the sulfur component. Therefore,the integration value includes almost no component of the sensor currentnot subject to the oxidizing reaction of the sulfur component.

In addition, the integration value is acquired by using the SOxconcentration currents.

In summary, (1) the base current and the SOx concentration currents usedfor acquiring the integration value by the SOx concentration apparatusaccording to the first invention, are the sensor currents subject to theoxidizing reaction of the sulfur component derived from the SOx, (2) thedifferences between the base current and each of the SOx concentrationcurrents used for acquiring the integration value by the SOxconcentration acquiring apparatus according to the first invention, arevalues which include almost no component of the sensor current notsubject to the oxidizing reaction of the sulfur component, and (3) theintegration value is a value acquired by using the SOx concentrationcurrents by the SOx concentration acquiring apparatus according to thefirst invention.

Therefore, when the SOx concentration changes, a proportion of change ofthe integration value is larger than a proportion of change of adifference between the base current not subject to the oxidizingreaction of the sulfur component derived from the SOx and the SOxconcentration current Thus, the integration value represents the changeof the SOx concentration explicitly. The SOx concentration acquiringapparatus according to the first invention acquires the SOxconcentration by using such an integration value. Thus, the SOxconcentration acquiring apparatus can acquire the SOx concentrationaccurately.

According to an aspect of the first invention, the electronic controlunit (90) may be further configured to set, as the third voltage(Vdown_end), the sensor voltage (Vss), at which all the sulfur componentis expected to be reoxidized while the electronic control unit (90)executes the reoxidation voltage decreasing control.

When the sensor voltage, at which all of the sulfur component isexpected to be oxidized, is set as the third voltage, the SOxconcentration acquiring apparatus according to this aspect can acquirethe sensor currents subject to the oxidizing reaction of the sulfurcomponent in a wide range of the sensor voltage. Thereby, the change ofthe integration value is large when the SOx concentration changes. Thus,the SOx concentration acquiring apparatus can acquire the SOxconcentration accurately.

According to another aspect of the first invention, the electroniccontrol unit (90) may be further configured to execute a constantvoltage control for controlling the sensor voltage (Vss) to a voltagelower than the oxygen increasing voltage (Vox_in) before the electroniccontrol unit (90) executes the decomposition voltage increasing controlafter the electronic control unit (90) executes the reoxidation voltagedecreasing control (see a step 980 in FIG. 9 and a step 1550 in FIG.15). In this case, the electronic control unit (90) may be furtherconfigured to acquire an oxygen concentration (Coxy) of the exhaust gason the basis of the sensor current (Iss_oxy) acquired while theelectronic control unit (90) executes the constant voltage control (seesteps 985 and 990 in FIG. 9 and steps 1560 and 1570 in FIG. 15).Thereby, the SOx concentration acquiring apparatus according to thisaspect can acquire the oxygen concentration of the exhaust gas as wellas the SOx concentration of the exhaust gas.

According to further another aspect of the first invention, the SOxconcentration acquiring apparatus may comprise the solid electrolytelayer (21A) as a first solid electrolyte layer. In this case, the SOxconcentration acquiring apparatus may further comprise a pump cell (25).The pump cell (25) may be formed by a second solid electrolyte layer(21B), a first pump electrode (25A), and a second pump electrode (25B).The first pump electrode (25A) may be provided on one of oppositesurfaces of the second solid electrolyte layer (21B) such that the firstpump electrode (25A) exposes to the exhaust gas. The second pumpelectrode (25B) may be provided on the other surface of the second solidelectrolyte layer (21B). In this case, the electronic control unit (90)may be further configured to execute a pump voltage control for applyinga voltage (Vpp) capable of decreasing an oxygen concentration of theexhaust gas to generally zero to the pump cell (25) and a constantvoltage control for controlling the sensor voltage (Vss) to a constantvoltage lower than the oxygen increasing voltage (Vox_in) (see a step2480 in FIG. 24). The electronic control unit (90) may be furtherconfigured to acquire a NOx concentration (Cnox) of the exhaust gas onthe basis of the sensor current (Iss_nox) acquired while the electroniccontrol unit (90) executes the pump voltage control and the constantvoltage control (see steps 2485 and 2487 in FIG. 24). Thereby, the SOxconcentration acquiring apparatus according to this aspect can acquirethe NOx concentration of the exhaust gas as well as the SOxconcentration of the exhaust gas.

According to further another aspect of the first invention, the SOxconcentration acquiring apparatus may further comprise a pump cell (25).The pump cell (25) may be formed by the solid electrolyte layer (11,21A), a first pump electrode (25A), and a second pump electrode (25B).The first pump electrode (25A) may be provided on one of the oppositesurfaces of the solid electrolyte layer (11, 21A) such that the firstpump electrode (25A) exposes to the exhaust gas. The second pumpelectrode (25B) may be provided on the other surface of the solidelectrolyte layer (11, 21A). In this case, the electronic control unit(90) may be further configured to execute a pump voltage control forapplying a voltage (Vpp) capable of decreasing an oxygen concentrationof the exhaust gas to generally zero to the pump cell (25) and aconstant voltage control for controlling the sensor voltage (Vss) to aconstant voltage lower than the oxygen increasing voltage (Vox_in). Theelectronic control unit (90) may be further configured to acquire a NOxconcentration (Cnox) of the exhaust gas on the basis of the sensorcurrent (Iss_oxy) acquired while the electronic control unit (90)executes the pump voltage control and the constant voltage control.Thereby, the SOx concentration acquiring apparatus according to thisaspect can acquire the NOx concentration of the exhaust gas as well asthe SOx concentration of the exhaust gas.

According to further another aspect of the first invention, theelectronic control unit (90) may be further configured to acquire anoxygen concentration (Coxy) of the exhaust gas on the basis of a pumpcurrent (Ipp_oxy) which is a current (Ipp) flowing through the pump cell(25) while the electronic control unit (90) executes the pump voltagecontrol (see the step 2485 and a step 2490 in FIG. 24). Thereby, the SOxconcentration acquiring apparatus according to this aspect can acquirethe oxygen concentration of the exhaust gas as well as the SOxconcentration and the NOx concentration of the exhaust gas.

A SOx concentration acquiring apparatus of an internal combustion engine(50) according to the second invention comprises a sensor cell (15, 26)and an electronic control unit (90). The sensor cell (15, 26) is formedby a solid electrolyte layer (11, 21A), a first sensor electrode (15A,26A), and a second sensor electrode (15B, 26B). The first sensorelectrode (15A, 25A) is provided on one of opposite surfaces of thesolid electrolyte layer (11, 21A) such that the first sensor electrode(11, 21A) exposes to an exhaust gas discharged from the internalcombustion engine (50). The second sensor electrode (15B, 26B) isprovided on the other surface of the solid electrolyte layer (11, 21A).

The electronic control unit (90) controls a sensor voltage (Vss) whichis a voltage applied to the sensor cell (15, 26) and acquires a sensorcurrent (Iss) which is a current flowing through the sensor cell (15,26).

The electronic control unit (90) is configured to execute adecomposition voltage increasing control for increasing the sensorvoltage (Vss) from a first voltage lower than an oxygen increasingvoltage (Vox_in) to a second voltage (Vup_end) equal to or higher thanthe oxygen increasing voltage (Vox_in) (see the step 920 in FIG. 9 andthe step 1710 in FIG. 17). The oxygen increasing voltage (Vox_in) is avoltage, at which an amount of oxygen component produced by SOxdecomposing to sulfur component and the oxygen component is larger thanthe amount of the oxygen component consumed by the sulfur componentbeing oxidized by the oxygen component to the SOx. The electroniccontrol unit (90) is further configured to execute a reoxidation voltagedecreasing control for decreasing the sensor voltage (Vss) from thesecond voltage (Vup_end) to a third voltage (Vdown_end) lower than anoxygen decreasing voltage (Vox_de) after the electronic control unit(90) executes the decomposition voltage increasing control (see the step940 in FIG. 9 and the step 1730 in FIG. 17). The oxygen decreasingvoltage (Vox_de) is a voltage, at which the amount of the oxygencomponent consumed by the sulfur component being oxidized by the oxygencomponent to the SOx is larger than the amount of the oxygen componentproduced by the SOx decomposing to the sulfur component and the oxygencomponent. The electronic control unit (90) is further configured toacquire the sensor currents (Iss) as SOx concentration currents(Iss_sox), respectively after the sensor voltage (Vss) reaches theoxygen decreasing voltage (Vox_de) while the electronic control unit(90) executes the reoxidation voltage decreasing control (see the step945 in FIG. 9). The electronic control unit (90) is further configuredto acquire the sensor current (Iss) as a high-voltage current (Ihigh)when the sensor voltage (Vss) decreases to a fourth voltage equal to orlower than the oxygen decreasing voltage (Vox_de) (see a step 1810 inFIG. 18). The electronic control unit (90) is further configured toacquire the sensor current (Iss) as a low-voltage current (Ilow) whenthe sensor voltage (Vss) decreases to a fifth voltage (Vdown_end) lowerthan the fourth voltage (see a step 1670 in FIG. 16). The electroniccontrol unit (90) is further configured to acquire a change rate of thesensor current (Iss) as a sensor current change rate (R) while thesensor current (Iss) changes from the high-voltage current (Ihigh) tothe low-voltage current (Ilow) (see a step 1820 in FIG. 18). Theelectronic control unit (90) is further configured to acquire currentswhich change from the high-voltage current (Ihigh) at the sensor currentchange rate (R) and correspond to the sensor voltages (Vss), at whichthe electronic control unit (90) acquires the SOx concentration currents(Iss_sox), as base currents (Ibase), respectively (see a step 1820 inFIG. 18). The electronic control unit (90) is further configured toacquire an integration value (S21, S22) of differences (dIss) betweeneach of the base currents (Ibase) and each of the SOx concentrationcurrents (Iss_sox) (see a step 1835 in FIG. 18). The electronic controlunit (90) is further configured to acquire a SOx concentration (Csox) ofthe exhaust gas on the basis of the integration value (S21, S22) (see astep 1840 in FIG. 18).

The sensor current change rate acquired on the basis of the high-voltagecurrent and the low-voltage current, is a value generally correspondingto a change rate of the sensor current after the sensor voltage reachesthe oxygen decreasing voltage when the SOx concentration of the exhaustgas is zero. In addition, the base currents are acquired from thecurrents changing from the high-voltage current at the sensor currentchange rate acquired on the basis of the high-voltage current and thelow-voltage current. Therefore, the differences between each of the basecurrents and each of the SOx concentration currents are values whichinclude almost no component of the sensor current not subject to theoxidizing reaction of the sulfur component. Therefore, when the SOxconcentration changes, a proportion of change of the integration valueis larger than a proportion of change of a difference between the basecurrent not subject to the oxidizing reaction of the sulfur componentderived from the SOx and the SOx concentration current. Thus, theintegration value represents a change of the SOx concentrationexplicitly. The SOx concentration acquiring apparatus according to thesecond invention acquires the SOx concentration by using such anintegration value. Thus, the SOx concentration acquiring apparatus canacquire the SOx concentration accurately.

According to an aspect of the second invention, the electronic controlunit (90) may be further configured to acquire the sensor current (Iss)as the high-voltage current (Ihigh) when the sensor voltage (Vss)reaches the fourth voltage (see a step 1730 in FIG. 17 and a step 1810in FIG. 18).

The SOx concentration acquiring apparatus according to this aspectacquires the high-voltage current while the SOx concentration acquiringapparatus executes the reoxidation voltage decreasing control. Thereoxidation voltage decreasing control is executed for acquiring the SOxconcentration currents used for acquiring the SOx concentration.Therefore, an additional control for decreasing the sensor voltage doesnot need to be executed for acquiring the high-voltage current. Thus,the SOx concentration acquiring apparatus can acquire the SOxconcentration for a short time.

Further, the SOx concentration acquiring apparatus according to thisaspect acquires the high-voltage current while the SOx concentrationacquiring apparatus executes the reoxidation voltage decreasing controlfor acquiring the SOx concentration currents. Therefore, conditionsrelating to the SOx concentration of the exhaust gas, the decomposingreaction of the SOx at the first sensor electrode, the oxidizingreaction of the sulfur component at the first sensor electrode, and thelike when the high-voltage current is acquired, are the same as thosewhen the SOx concentration currents are acquired. Thus, the SOxconcentration acquiring apparatus can acquire the high-voltage currentsuitable for acquiring the SOx concentration accurately. Therefore, theSOx concentration acquiring apparatus can acquire the SOx concentrationaccurately.

According to another aspect of the second invention, the electroniccontrol unit (90) may be further configured to set the oxygen decreasingvoltage (Vox_de) as the fourth voltage (see a step 1810 in FIG. 18).

The SOx concentration acquiring apparatus according to this aspectacquires the sensor current as the high-voltage current when the sensorvoltage reaches the oxygen decreasing voltage. The oxygen decreasingvoltage is the sensor voltage, at which the amount of the oxygencomponent consumed by the sulfur component being oxidized to the SOx islarger than the amount of the oxygen component produced by the SOxdecomposing to the sulfur component and the oxygen component. Therefore,the differences between each of the base currents and each of the SOxconcentration currents include the component of the sensor currentsubject to the oxidizing reaction of the sulfur component to a largeextent. As a result, the change of the integration value is large whenthe SOx concentration changes. Therefore, the integration valuerepresents the change of the SOx concentration explicitly. The SOxconcentration acquiring apparatus according to this aspect acquires theSOx concentration by using such an integration value. Thus, the SOxconcentration acquiring apparatus can acquire the SOx concentrationaccurately.

According to further another aspect of the second invention, theelectronic control unit (90) may be further configured to execute apreliminary voltage increasing control for increasing the sensor voltage(Vss) to a sixth voltage lower than the fourth voltage and the oxygendecreasing voltage (Vox_de) (see a step 1635 in FIG. 16). In this case,the electronic control unit (90) may be further configured to execute apreliminary voltage decreasing control for decreasing the sensor voltage(Vss) from the sixth voltage to a voltage equal to or lower than thefifth voltage after the electronic control unit (90) executes thepreliminary voltage increasing control (see a step 1655 in FIG. 16). Theelectronic control unit (90) may be further configured to acquire thesensor current (Iss) as the low-voltage current (Ilow) when the sensorvoltage (Vss) reaches the fifth voltage while the electronic controlunit (90) executes the preliminary voltage decreasing control (see astep 1670 in FIG. 16).

The low-voltage current is used for acquiring the sensor current changerate. The sensor current change rate is desirably near the change rateof the SOx concentration current with the SOx concentration of theexhaust gas being zero. Therefore, the low-voltage current is desirablythe sensor current acquired when the sensor voltage reaches the fifthvoltage with the SOx concentration of the exhaust gas being zero. Thatis, the low-voltage current is desirably the sensor current not subjectto the decomposing reaction of the SOx and the oxidizing reaction of thesulfur component.

While the preliminary voltage increasing control and the preliminaryvoltage decreasing control are executed, the sensor voltage changeswithin a range of the sensor voltage lower than the oxygen increasingvoltage. Therefore, while the preliminary voltage increasing control andthe preliminary voltage decreasing control are executed, almost no SOxdecomposes and almost no sulfur component is oxidized. Therefore, ingeneral, the sensor current acquired while the preliminary voltagedecreasing control is executed, is not subject to the decomposingreaction of the SOx and the oxidizing reaction of the sulfur component.Therefore, the SOx concentration acquiring apparatus according to thisaspect can acquire the sensor current change rate near the change rateof the SOx concentration current with the SOx concentration of theexhaust gas being zero by acquiring the sensor current as thelow-voltage current when the sensor voltage reaches the fifth voltagewhile the SOx concentration acquiring apparatus executes the preliminaryvoltage decreasing control.

According to further another aspect of the second invention, theelectronic control unit (90) may be further configured to acquire thesensor current (Iss) as the low-voltage current (Ilow) when the sensorvoltage (Vss) reaches the fifth voltage while the electronic controlunit (90) executes the reoxidation voltage decreasing control (see astep 2080 in FIG. 20).

The SOx concentration acquiring apparatus according to this aspectacquires the low-voltage current while the SOx concentration acquiringapparatus executes the reoxidation voltage decreasing control. Thereoxidation voltage decreasing control is executed for acquiring the SOxconcentration currents used for acquiring the SOx concentration.Therefore, an additional control for decreasing the sensor voltage doesnot need to be executed for acquiring the low-voltage current. Thus, theSOx concentration acquiring apparatus can acquire the SOx concentrationfor a short time.

Further, the SOx concentration acquiring apparatus according to thisaspect acquires the low-voltage current while the SOx concentrationacquiring apparatus executes the reoxidation voltage decreasing controlfor acquiring the SOx concentration currents. Therefore, conditionsrelating to the SOx concentration of the exhaust gas, the decomposingreaction of the SOx at the first sensor electrode, the oxidizingreaction of the sulfur component at the first sensor electrode, and thelike when the low-voltage current is acquired, are the same as thosewhen the SOx concentration currents are acquired. Thus, the SOxconcentration acquiring apparatus can acquire the high-voltage currentsuitable for acquiring the SOx concentration accurately. Therefore, theSOx concentration acquiring apparatus can acquire the SOx concentrationaccurately.

According to further another aspect of the second invention, theelectronic control unit (90) may be further configured to set, as thethird voltage (Vdown_end), the sensor voltage, at which all the sulfurcomponent is expected to be reoxidized while the electronic control unit(90) executes the reoxidation voltage decreasing control.

When the sensor voltage, at which all of the sulfur component isexpected to be oxidized, is set as the third voltage, the SOxconcentration acquiring apparatus according to this aspect can acquirethe sensor currents subject to an oxidizing reaction of the sulfurcomponent as the SOx concentration currents in a wide range of thesensor voltage. Thereby, the change of the integration value is largewhen the SOx concentration changes. Thus, the SOx concentrationacquiring apparatus can acquire the SOx concentration accurately.

According to further another aspect of the second invention, theelectronic control unit (90) may be further configured to execute aconstant voltage control for controlling the sensor voltage (Vss) to avoltage lower than the oxygen increasing voltage (Vox_in) before theelectronic control unit (90) executes the decomposition voltageincreasing control after the electronic control unit (90) executes thereoxidation voltage decreasing control (see the step 980 in FIG. 9 andthe step 1550 in FIG. 15). In this case, the electronic control unit(90) may be further configured to acquire an oxygen concentration (Coxy)of the exhaust gas on the basis of the sensor current (Iss_oxy) acquiredwhile the electronic control unit (90) executes the constant voltagecontrol (see the steps 985 and 990 in FIG. 9 and the steps 1560 and 1570in FIG. 15). Thereby, the SOx concentration acquiring apparatusaccording to this aspect can acquire the oxygen concentration of theexhaust gas as well as the SOx concentration of the exhaust gas.

According to further another aspect of the second invention, the SOxconcentration acquiring apparatus may comprise the solid electrolytelayer (21A) as a first solid electrolyte layer. In this case, the SOxconcentration acquiring apparatus may further comprise a pump cell (25).The pump cell (25) may be formed by a second solid electrolyte layer(21B), a first pump electrode (25A), and a second pump electrode (25B).The first pump electrode (25A) may be provided on one of oppositesurfaces of the second solid electrolyte layer (21B) such that the firstpump electrode (25A) exposes to the exhaust gas. The second pumpelectrode (25B) may be provided on the other surface of the second solidelectrolyte layer (21B). In this case, the electronic control unit (90)may be further configured to execute a pump voltage control for applyinga voltage (Vpp) capable of decreasing an oxygen concentration of theexhaust gas to generally zero to the pump cell (25) and a constantvoltage control for controlling the sensor voltage (Vss) to a constantvoltage lower than the oxygen increasing voltage (Vox_in) (see the step2480 in FIG. 24). The electronic control unit (90) may be furtherconfigured to acquire a NOx concentration (Cnox) of the exhaust gas onthe basis of the sensor current (Iss_nox) acquired while the electroniccontrol unit (90) executes the pump voltage control and the constantvoltage control (see the steps 2485 and 2487 in FIG. 24). Thereby, theSOx concentration acquiring apparatus according to this aspect canacquire the NOx concentration of the exhaust gas as well as the SOxconcentration of the exhaust gas.

According to further another aspect of the second invention, the SOxconcentration acquiring apparatus may further comprise a pump cell (25).The pump cell (25) may be formed by the solid electrolyte layer (11,21A), a first pump electrode (25A), and a second pump electrode (25B).The first pump electrode (25A) may be provided on one of the oppositesurfaces of the solid electrolyte layer (11, 21A) such that the firstpump electrode (25A) exposes to the exhaust gas. The second pumpelectrode (25B) may be provided on the other surface of the solidelectrolyte layer (11, 21A). In this case, the electronic control unit(90) may be further configured to execute a pump voltage control forapplying a voltage (Vpp) capable of decreasing an oxygen concentrationof the exhaust gas to generally zero to the pump cell (25) and aconstant voltage control for controlling the sensor voltage (Vss) to aconstant voltage lower than the oxygen increasing voltage (Vox_in). Theelectronic control unit (90) may be further configured to acquire a NOxconcentration (Cnox) of the exhaust gas on the basis of the sensorcurrent (Iss) acquired while the electronic control unit (90) executesthe pump voltage control and the constant voltage control. Thereby, theSOx concentration acquiring apparatus according to this aspect canacquire the NOx concentration of the exhaust gas as well as the SOxconcentration of the exhaust gas.

According to further another aspect of the second invention, theelectronic control unit (90) may be further configured to acquire anoxygen concentration (Coxy) of the exhaust gas on the basis of a pumpcurrent (Ipp_oxy) which is a current (Ipp) flowing through the pump cell(25) while the electronic control unit (90) executes the pump voltagecontrol (see the steps 2485 and 2490 in FIG. 24). Thereby, the SOxconcentration acquiring apparatus according to this aspect can acquirethe oxygen concentration of the exhaust gas as well as the SOxconcentration and the NOx concentration of the exhaust gas.

In the above description, for facilitating understanding of the presentinvention, elements of the present invention corresponding to elementsof an embodiment described later are denoted by reference symbols usedin the description of the embodiment accompanied with parentheses.However, the elements of the present invention are not limited to theelements of the embodiment defined by the reference symbols. The otherobjects, features and accompanied advantages of the present inventioncan be easily understood from the description of the embodiment of thepresent invention along with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for showing an internal combustion engine provided witha SOx concentration acquiring apparatus according to a first embodimentof the invention.

FIG. 2 is a view for showing an inner configuration of a limitingcurrent sensor of the SOx concentration acquiring apparatus according tothe first embodiment.

FIG. 3 is a view for showing a relationship among a voltage applied to asensor cell of the limiting current sensor of the SOx concentrationacquiring apparatus according to the first embodiment, a current flowingthrough the sensor cell, and an oxygen concentration of the exhaust gasjust discharged from the internal combustion engine.

FIG. 4A is a view for showing a relationship between the voltage appliedto the sensor cell and the current flowing through the sensor cell.

FIG. 4B is a view for showing a relationship between the voltage appliedto the sensor cell and the current flowing through the sensor cell.

FIG. 5 is a view for showing a relationship between the voltage appliedto the sensor cell and the current flowing through the sensor cell.

FIG. 6 is a view for showing a relationship between a current differenceintegration value and a SOx concentration of the exhaust gas justdischarged from the internal combustion engine.

FIG. 7 is a view for showing a time chart illustrating changes of thevoltage applied to the sensor cell.

FIG. 8 is a view for showing manners of increasing and decreasing thevoltage applied to the sensor cell by the SOx concentration acquiringapparatus according to the first embodiment.

FIG. 9 is a view for showing a flowchart illustrating a routine executedby a CPU of an ECU of the SOx concentration acquiring apparatusaccording to the first embodiment.

FIG. 10 is a view for showing a flowchart illustrating a routineexecuted by the CPU.

FIG. 11 is a view for showing a flowchart illustrating a routineexecuted by the CPU.

FIG. 12 is a view for showing a relationship between the voltage appliedto the sensor cell of the limiting current sensor of the SOxconcentration acquiring apparatus according to a modified example of thefirst embodiment and the current flowing through the sensor cell of themodified example.

FIG. 13 is a view for showing a relationship between the voltage appliedto the sensor cell of the modified example and the current flowingthrough the sensor cell of the modified example.

FIG. 14A is a view for showing a relationship between the voltageapplied to the sensor cell of the modified example and the currentflowing through the sensor cell of the modified example.

FIG. 14B is a view for showing a relationship between the voltageapplied to the sensor cell of the modified example and the currentflowing through the sensor cell of the modified example.

FIG. 15 is a view for showing a flowchart illustrating a routineexecuted by the CPU of the ECU of the SOx concentration acquiringapparatus according to the modified example.

FIG. 16 is a view for showing a flowchart illustrating a routineexecuted by the CPU of the modified example.

FIG. 17 is a view for showing a flowchart illustrating a routineexecuted by the CPU of the modified example.

FIG. 18 is a view for showing a flowchart illustrating a routineexecuted by the CPU of the modified example.

FIG. 19 is a view for showing a flowchart illustrating a routineexecuted by the CPU of the modified example.

FIG. 20 is a view for showing a flowchart illustrating a routineexecuted by the CPU of the modified example.

FIG. 21 is a view for showing the internal combustion engine providedwith the SOx concentration acquiring apparatus according to a secondembodiment of the invention.

FIG. 22 is a view for showing an inner configuration of a limitingcurrent sensor of the SOx concentration acquiring apparatus according tothe second embodiment.

FIG. 23 is a view for showing a relationship between the current flowingthrough the sensor cell of the sensor of the SOx concentration acquiringapparatus according to the second embodiment and a NOx concentration ofthe exhaust gas just discharged from the internal combustion engine.

FIG. 24 is a view for showing a flowchart illustrating a routineexecuted by the CPU of the ECU of the SOx concentration acquiringapparatus according to the second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, a SOx concentration acquiring apparatus of an internal combustionengine according to embodiments of the invention will be described withreference to the drawings. The SOx concentration acquiring apparatusaccording to a first embodiment of the invention is applied to theinternal combustion engine 50 shown in FIG. 1. Hereinafter, the SOxconcentration acquiring apparatus according to the first embodiment willbe referred to as “the first embodiment apparatus”.

The internal combustion engine 50 is a spark-ignition internalcombustion engine (i.e., a so-called gasoline engine). In thisconnection, the invention may be applied to a compression-ignitioninternal combustion engine (i.e., a so-called diesel engine). Theinternal combustion engine 50 shown in FIG. 1 operates at astoichiometric air-fuel ratio in a substantial engine operation region.

In FIG. 1, a reference sign 51 denotes a cylinder head, 52 denotes acylinder block, 53 denotes combustion chambers, 54 denotes fuelinjectors, 55 denotes spark plugs, 56 denotes a fuel pump, 57 denotes afuel supply pipe, 60 denotes pistons, 61 denotes connecting rods, 62denotes a crank shaft, 63 denotes a crank angle sensor, 70 denotesintake valves, 71 denotes intake ports, 72 denotes an intake manifold,73 denotes a surge tank, 74 denotes a throttle valve, 75 denotes anintake pipe, 76 denotes an air-flow meter, 77 denotes an air filter, 80denotes exhaust valves, 81 denotes exhaust ports, 82 denotes an exhaustmanifold, 83 denotes an exhaust pipe, 90 denotes an electronic controlunit, 91 denotes an acceleration pedal, and 92 denotes an accelerationpedal operation amount sensor. Hereinafter, the electronic control unit90 will be referred to as “the ECU 90”.

The fuel injectors 54, the ignition plugs 55, the throttle valve 74, thecrank angle sensor 63, the air-flow meter 76, the acceleration pedaloperation amount sensor 92, and a limiting current sensor 10 areelectrically connected to the ECU 90.

The ECU 90 is an electronic control circuit including as a maincomponent a microcomputer including a CPU, a ROM, a RAM, an interface,etc. The CPU realizes various functions by executing instructions orroutines stored in a memory (i.e., the ROM).

The ECU 90 is configured to send signals to the fuel injectors 54, theignition plugs 55, and the throttle valve 74 for activating the fuelinjectors 54, the ignition plugs 55, and the throttle valve 74,respectively. The ECU 90 receives signals from the crank angle sensor63, the air-flow meter 76, and the acceleration pedal operation amountsensor 92. The crank angle sensor 63 outputs a signal corresponding to arotation speed of the crank shaft 62. The ECU 90 calculates an enginespeed (i.e., a rotation speed of the internal combustion engine 50) onthe basis of the signals output from the crank angle sensor 63. Theair-flow meter 76 outputs a signal corresponding to a flow rate of anair passing the air-flow meter 76, that is, a flow rate of the airflowing into the combustion chambers 53. The ECU 90 calculates an intakeair amount (i.e., an amount of the air flowing into the combustionchambers 53) on the basis of the signals output from the air-flow meter76. The acceleration pedal operation amount sensor 92 outputs a signalcorresponding to an operation amount of the acceleration pedal 91. TheECU 90 calculates an engine load KL (i.e., a load of the internalcombustion engine 50) on the basis of the signals output from theacceleration pedal operation amount sensor 92.

The first embodiment apparatus includes the limiting current sensor 10,a sensor cell voltage source 15C, a sensor cell ammeter 15D, a sensorcell voltmeter 15E, and the ECU 90. The sensor 10 is a single-cell typelimiting current sensor. The sensor 10 is provided on the exhaust pipe83.

As shown in FIG. 2, the sensor 10 includes a solid electrolyte layer 11,a first alumina layer 12A, a second alumina layer 12B, a third aluminalayer 12C, a fourth alumina layer 12D, a fifth alumina layer 12E, adiffusion-limited layer 13, a heater 14, a sensor cell 15, a firstsensor electrode 15A, a second sensor electrode 15B, an atmospheric airintroduction passage 16, and an interior space 17.

The solid electrolyte layer 11 is a layer formed of zirconia or the likeand has oxygen ion conductive property. The alumina layers 12A to 12Eare layers formed of alumina, respectively. The diffusion-limited layer13 is a porous layer, through which an exhaust gas discharged from thecombustion chambers 53 of the engine 50 can flow. In the sensor 10, thelayers are laminated such that the fifth alumina layer 12E, the fourthalumina layer 12D, the third alumina layer 12C, the solid electrolytelayer 11, the diffusion-limited layer 13 and the second alumina layer12B, and the first alumina layer 12A are positioned in order from thelower side of FIG. 2. The heater 14 is positioned between the fourth andfifth alumina layers 12D and 12E.

The atmospheric air introduction passage 16 is a space defined by thesolid electrolyte layer 11, the third alumina layer 12C, and the fourthalumina layer 12D, and a part of the atmospheric air introductionpassage 16 opens to the atmosphere. The interior space 17 is a spacedefined by the first alumina layer 12A, the solid electrolyte layer 11,the diffusion-limited layer 13, and the second alumina layer 12B, and apart of the interior space 17 communicates with the outside of thesensor 10 via the diffusion-limited layer 13.

The first and second sensor electrodes 15A and 15B are electrodes formedof material having a high reducing property, for example, platinum groupelement such as platinum and rhodium or alloy of the platinum groupelement. The first sensor electrode 15A is positioned on one of oppositesurfaces of the solid electrolyte layer 11 (that is, the surface of thesolid electrolyte layer 11 which defines the interior space 17). Thesecond sensor electrode 15B is positioned on the other surface of thesolid electrolyte layer 11 (that is, the surface of the solidelectrolyte layer 11 which defines the atmospheric air introductionpassage 16). The first sensor electrode 15A, the second sensor electrode15, and the solid electrolyte layer 11 form the sensor cell 15.

The exhaust gas discharged from the engine 50 flows into the interiorspace 17 through the diffusion-limited layer 13. The first sensorelectrode 15A exposes to the exhaust gas flowing into the interior space17.

The sensor 10 is configured to be able to apply a voltage from thesensor cell voltage source 15C to the sensor cell 15 (in particular, tothe second sensor electrode 15B so as to produce an electric potentialdifference with respect to the first sensor electrode 15A). The sensorcell voltage source 15C is configured to apply a direct voltage to thesensor cell 15. It should be noted that the first sensor electrode 15Ais a cathode side electrode, and the second sensor electrode 15B is ananode side electrode when the sensor cell voltage source 15C applies thedirect voltage to the sensor cell 15.

The heater 14, the sensor cell voltage source 15C, the sensor cellammeter 15D, and the sensor cell voltmeter 15E are electricallyconnected to the ECU 90.

The ECU 90 controls an activation of the heater 14 to maintain atemperature of the sensor cell 15 at a sensor activating temperature, atwhich the sensor 10 is activated.

In addition, the ECU 90 controls a voltage of the sensor cell voltagesource 15C to apply a voltage set as described later to the sensor cell15 from the sensor cell voltage source 15C.

The sensor cell ammeter 15D detects a current Iss flowing through acircuit including the sensor cell 15 and outputs a signal representingthe detected current Iss to the ECU 90. The ECU 90 acquires the currentIss on the basis of the signal. Hereinafter, the current Iss will bereferred to as “the sensor current Iss”.

The sensor cell voltmeter 15E detects a voltage Vss applied to thesensor cell 15 and outputs a signal representing the detected voltageVss to the ECU 90. The ECU 90 acquires the voltage Vss on the basis ofthe signal. Hereinafter, the voltage Vss will be referred to as “thesensor voltage Vss”.

Summary of Operation of First Embodiment Apparatus Acquisition ofExhaust SOx Concentration

When the voltage is applied to the sensor cell 15, and SOx (i.e., sulfuroxide) included in the exhaust gas flowing into the interior space 17contacts the first sensor electrode 15A, the SOx is reduced anddecomposed on the first sensor electrode 15A, oxygen component of theSOx becomes oxygen ion and then, the oxygen ion moves toward the secondsensor electrode 15B through the solid electrolyte layer 11. At thistime, an electric current proportional to an amount of the oxygen ion,which has moved through the solid electrolyte layer 11, flows betweenthe first and second sensor electrodes 15A and 15B. Then, when theoxygen ion reaches the second sensor electrode 15B, the oxygen ionbecomes oxygen on the second sensor electrode 15B and then, isdischarged to the atmospheric air introduction passage 16.

A relationship among the sensor voltage Vss, the sensor current Iss, andan air-fuel ratio A/F of the exhaust gas just discharged from the engine50, is shown in FIG. 3. The sensor voltage Vss is a direct voltageapplied to the sensor cell 15 by the sensor cell voltage source 15C. Thesensor current Iss is an electric current flowing between the first andsecond sensor electrodes 15A and 15B when the direct voltage is appliedto the sensor cell 15. The air-fuel ratio A/F of the exhaust gascorresponds to an air-fuel ratio of a mixture formed in the combustionchambers 53. Hereinafter, the air-fuel ratio A/F of the exhaust gas willbe referred to as “the exhaust air-fuel ratio A/F”.

In FIG. 3, a line denoted by A/F=12 shows a change of the sensor currentIss relative to a change of the sensor voltage Vss in case that theexhaust gas air-fuel ratio A/F is 12. Similarly, lines denoted by A/F=13to A/F=18 show changes of the sensor current Iss relative to changes ofthe sensor voltage Vss in case that the exhaust air-fuel ratios A/F are13 to 18, respectively.

For example, in case that the exhaust gas air-fuel ratio A/F is 18, andthe sensor voltage Vss is within a range lower than a predeterminedvalue Vth, when the sensor current Iss is a negative value, an absolutevalue of the sensor current Iss decreases as the sensor voltage Vssincreases. On the other hand, when the sensor current Iss is a positivevalue, the absolute value of the sensor current Iss increases as thesensor voltage Vss increases. Further, in case that the sensor voltageVss is within a constant range higher than or equal to the predeterminedvalue Vth, the sensor current Iss is a constant value, independently ofthe sensor voltage Vss. Hereinafter, the predetermined value Vth will bereferred to as “the limiting current range lower limit voltage”.

Similarly, this relationship between the sensor voltage Vss and thesensor current Iss is established in case that the exhaust gas air-fuelratios A/F are 12 to 17, respectively.

From a study, the inventors of this application have a new knowledgethat the sensor current Iss changes as shown in FIG. 4A while graduallyincreasing the sensor voltage Vss from 0.2 V to 0.8 V and then,gradually decreasing the sensor voltage Vss from 0.8 V to 0.2 V when theexhaust gas including no SOx and having a constant oxygen concentration,reaches the first sensor electrode 15A.

As shown by a line LU1 in FIG. 4A, the sensor current Iss is about 0.4mA when the sensor voltage Vss is 0.2 V. When the sensor voltage Vssstarts to increase from 0.2 V, the sensor current Iss starts to increasefrom about 0.4 mA. While the sensor voltage Vss increases to about 0.35V, the sensor current Iss increases rapidly. When the sensor voltage Vssreaches about 0.35 V, the sensor current Iss starts to decrease. Whilethe sensor voltage Vss reaches about 0.6 V after the sensor voltage Vssreaches about 0.35 V, the sensor current Iss decreases slightly. Whenthe sensor voltage Vss reaches about 0.6 V, the sensor current Issstarts to increase. While the sensor voltage Vss increases to about 0.75V after the sensor voltage Vss reaches about 0.6 V, the sensor currentIss increases slightly. When the sensor voltage Vss reaches about 0.75V, the sensor current Iss starts to decrease. When the sensor voltageVss reaches 0.8 V, the sensor current Iss reaches about 0.55 mA.

When the sensor voltage Vss starts to decrease from 0.8 V, the sensorcurrent Iss starts to decrease from about 0.5 mA as shown by a line LD1in FIG. 4A. While the sensor voltage Vss decreases after the sensorvoltage Vss reaches about 0.6 V, the sensor current Iss is generallyconstant at a current slightly higher than 0.4 mA. While the sensorvoltage Vss decreases after the sensor voltage Vss reaches about 0.5 V,the sensor current Iss decreases. After the sensor voltage Vss reaches avoltage slightly higher than 0.2 V, the sensor current Iss increases.When the sensor voltage Vss reaches 0.2 V, the sensor current Issreaches about 0.4 mA.

On the other hand, the inventors of this application have a newknowledge that the sensor current Iss changes as shown in FIG. 4B whilegradually increasing the sensor voltage Vss from 0.2 V to 0.8 V andthen, gradually decreasing the sensor voltage Vss from 0.8 V to 0.2 Vwhen the exhaust gas including the SOx and having the constant oxygenconcentration reaches the first sensor electrode 15A.

Similar to an example shown in FIG. 4A, as shown by a line LU1 in FIG.4B, when the sensor voltage Vss is 0.2 V, the sensor current Iss isabout 0.4 mA. When the sensor voltage Vss starts to increase from 0.2 V,the sensor current Iss starts to increase from about 0.4 mA. The sensorcurrent Iss increases rapidly while the sensor voltage Vss increases toabout 0.35 V. When the sensor voltage Vss reaches about 0.35 V, thesensor current Iss starts to decrease. While the sensor voltage Vssincreases to about 0.6 V after the sensor voltage Vss reaches about 0.35V, the sensor current Iss decreases moderately. When the sensor voltageVss reaches about 0.6 V, the sensor current Iss starts to increase.While the sensor voltage Vss increases to about 0.75 V after the sensorvoltage Vss reaches about 0.6 V, the sensor current Iss increasesmoderately. When the sensor voltage Vss reaches about 0.7 V, the sensorcurrent Iss starts to decrease. When the sensor voltage Vss reaches 0.8V, the sensor current Iss reaches about 0.55 mA.

When the sensor voltage Vss starts to decrease from 0.8 V, the sensorcurrent Iss starts to decrease from about 0.55 mA as shown by a line LD1in FIG. 4B. While the sensor voltage Vss decreases to about 0.3 V, thesensor current Iss continues to decrease. While the sensor voltage Vssincreases to about 0.6 V, and the sensor current Iss continues todecrease, a decreasing rate of the sensor current Iss decreases. Afterthe sensor voltage Vss reaches about 0.6 V, the decreasing rate of thesensor current Iss increases gradually. When the sensor voltage Vssreaches about 0.3 V, the sensor current Iss reaches a minimum value ofabout 0.31 mA and starts to increase. When the sensor voltage Vssreaches 0.2 V, the sensor current Iss reaches about 0.4 mA.

The change of the sensor current Iss shown in FIG. 4B while the sensorvoltage Vss decreases from 0.8 V to 0.2 V when the exhaust gas includingthe SOx reaches the first sensor electrode 15A, is different from thechange of the sensor current Iss shown in FIG. 4A while the sensorvoltage Vss decreases from 0.8 V to 0.2 V when the exhaust gas includingno SOx reaches the first sensor electrode 15A.

In particular, the sensor current Iss while the sensor voltage Vssdecreases from 0.8 V to 0.2 V when the exhaust gas including the SOxreaches the first sensor electrode 15A, is generally lower than thesensor current Iss while the sensor voltage Vss decreases from 0.8 V to0.2 V when the exhaust gas including no SOx reaches the first sensorelectrode 15A.

In the sensor 10, there is a phenomenon that the sensor current Iss islow while the sensor voltage Vss decreases from 0.8 V to 0.2 V when theexhaust gas includes the SOx, compared with when the exhaust gasincludes no SOx. The inventors of this application have understoodreasons for the phenomenon as described below.

When the sensor voltage Vss exceeds a certain value while the sensorvoltage Vss increases from 0.2 V to 0.8 V, the SOx reaching the firstsensor electrode 15A decomposes to sulfur component and oxygen componentat the first sensor electrode 15A. The oxygen component changes to theoxygen ion and moves toward the second sensor electrode 15B through thesolid electrolyte layer 11. The sulfur component adheres to the firstsensor electrode 15A.

When the sensor voltage Vss decreases below a certain value while thesensor voltage Vss decreases from 0.8 V to 0.2 V, the sulfur componentadhering to the first sensor electrode 15A is oxidized by the oxygencomponent around the first sensor electrode 15A, thereby returning tothe SOx. At this time, a decomposing reaction of the SOx to the sulfurcomponent and the oxygen component at the first sensor electrode 15A,may occur. However, an oxidizing reaction of the sulfur componentadhering to the first sensor electrode 15A, is more dominant than thedecomposing reaction. As a result, an amount of the oxygen componentconsumed by the oxidizing reaction in the interior space 17 is largerthan an amount of the oxygen component produced from the SOx by thedecomposing reaction. Thus, the amount of the oxygen ion moving towardthe second sensor electrode 15B through the solid electrolyte layer 11decreases. Therefore, the sensor current Iss decreases. Thus, the sensorcurrent Iss is low while the sensor voltage Vss decreases from 0.8 V to0.2 V when the exhaust gas includes the SOx, compared with when theexhaust gas includes no SOx.

In this embodiment, the voltage of 0.8 V is employed as a voltagesuitable for causing a decomposing amount of the SOx at the first sensorelectrode 15A to reach a large amount sufficient for acquiring aconcentration of the SOx included in the exhaust gas just dischargedfrom the engine 50 exactly while the sensor voltage Vss increases from0.2 V to 0.8 V. Hereinafter, the concentration of the SOx will bereferred to as “the SOx concentration”, and the concentration of the SOxincluded in the exhaust gas just discharged from the engine 50 will bereferred to as “the exhaust SOx concentration”. Further, the sensorvoltage Vss at a point of time when the sensor voltage Vss stops toincrease, in this embodiment, the voltage of 0.8 V, will be referred toas “the increasing end voltage Vup_end”. The increasing end voltageVup_end is, for example, a voltage capable of causing reactions such asa decomposing reaction of water included in the exhaust gas at the firstsensor electrode 15A other than the decomposing reaction of the SOx tooccur to the minimum extent.

Further, in this embodiment, the voltage of 0.2 V is employed as avoltage suitable for causing an oxidizing amount of the sulfur componentadhering to the first sensor electrode 15A to reach a large amountsufficient for acquiring the exhaust SOx concentration exactly while thesensor voltage Vss decreases from 0.8 V to 0.2 V. Hereinafter, thesensor voltage Vss at point of time when the sensor voltage Vss stops todecrease, in this embodiment, the voltage of 0.2 V, will be referred toas “the decreasing end voltage Vdown_end”.

The inventors of this application have a following knowledge based onthe above description. In the following description, sign “m” of sign“(m)” represents an integer which increases from “1”. For example, thesensor current Iss(1) represents the first-acquired sensor current Iss,the sensor current Iss(2) represents the second-acquired sensor currentIss, and the sensor current Iss(m) represents the m-th-acquired sensorcurrent Iss. Sign “n” of sign “(n)” represents optional one of theintegers of 1 to m. Therefore, the sensor current Iss(n) represents oneof the sensor currents Iss(1) to Iss(m).

The inventors of this application increased the sensor voltage Vss froma voltage lower than an oxygen increasing voltage Vox_in to a voltagehigher than the oxygen increasing voltage Vox_in. The oxygen increasingvoltage Vox_in is the sensor voltage Vss for causing an amount of theoxygen component produced by the decomposing reaction of the SOx to thesulfur component and the oxygen component to become larger than theamount of the oxygen component consumed by the oxidizing of the sulfurcomponent to the SOx.

Then, the inventors of this application decreased the sensor voltage Vssfrom the voltage higher than the oxygen increasing voltage Vox_in to avoltage lower than an oxygen decreasing voltage Vox_de. The oxygendecreasing voltage Vox_de is the sensor voltage Vss for causing theamount of the oxygen component consumed by the oxidizing of the sulfurcomponent to the SOx to become larger than the amount of the oxygencomponent produced by the decomposing reaction of the SOx to the sulfurcomponent and the oxygen component. The inventors acquired the sensorcurrent Iss as a base current Ibase when the sensor voltage Vss reachedthe oxygen decreasing voltage Vox_de after the sensor voltage Vssstarted to decrease. In addition, the inventors acquired the sensorcurrents Iss(1) to Iss(m) after the sensor voltage Vss reached theoxygen decreasing voltage Vox_de.

Then, the inventors of this application acquired an integration valueS11 of differences dIss(n) between each of the acquired sensor currentsIss(n), that is, the acquired sensor currents Iss(1) to Iss(m) and thebase current Ibase (dIss(n)=Ibase−Iss(n)). The integration value S11corresponds to an area shown by a reference sign A11 in FIG. 5. As shownin FIG. 6, the exhaust SOx concentration increases as the integrationvalue S11 increases.

Accordingly, as shown in FIG. 7, the first embodiment apparatus executesa constant voltage control for controlling the sensor voltage Vss tomaintain the sensor voltage Vss at a constant value lower than theoxygen increasing voltage Vox_in when the exhaust SOx concentration isnot requested to be acquired, that is, in a time period before a pointof time t0. In this embodiment, the constant value lower than the oxygenincreasing voltage Vox_in is 0.4 V.

When the exhaust SOx concentration is requested to be acquired and anengine operation (that is, an operation of the engine 50) is in a steadyoperation state or an idling operation state, the first embodimentapparatus executes a concentration acquisition voltage control includinga decomposition voltage increasing control and a reoxidation voltagedecreasing control described below.

The exhaust SOx concentration is requested to be acquired, for example,when a vehicle equipped with the engine 50 moves for a predetermineddistance after fuel is supplied to a fuel tank which stores the fuel tobe supplied to the fuel injectors 54. Alternatively, the exhaust SOxconcentration is requested to be acquired when the vehicle equipped withthe engine 50 moves for the predetermined distance after the fuel issupplied to the fuel tank and thereafter, the exhaust SOx concentrationis requested to be acquired each time the vehicle moves for thepredetermined distance or another predetermined distance.

The steady operation state is a state that the engine speed NE and theengine load KL are constant or generally constant, respectively. Thatis, when the engine operation is in the steady operation state, aconcentration of the oxygen included in the exhaust gas just dischargedfrom the engine 50 is constant or generally constant. Hereinafter, theconcentration of the oxygen included in the exhaust gas just dischargedfrom the engine 50 will be referred to as “the exhaust oxygenconcentration”. The idling operation state is a state that the operationamount AP of the acceleration pedal is zero and thus, a minimum amountof the air required to maintain the operation of the engine 50 is causedto flow into the combustion chambers 53, and the fuel injectors 54 arecaused to inject the fuel. Therefore, the exhaust oxygen concentrationis constant or generally constant when the engine operation is in theidling operation state.

When the first embodiment apparatus starts to execute the concentrationacquisition voltage control, the first embodiment apparatus starts toexecute the decomposition voltage increasing control for increasing thesensor voltage Vss from 0.4 V with an increasing rate of the sensorvoltage Vss decreasing gradually (see the point of time t0 in FIG. 7).When the sensor voltage Vss reaches the increasing end voltage Vup_end(in this embodiment, 0.8 V), the first embodiment apparatus stopsexecuting the decomposition voltage increasing control (see a point oftime t1 in FIG. 7). Thereby, the first embodiment apparatus increasesthe sensor voltage Vss from 0.4 V to 0.8 V.

Thereafter, the first embodiment apparatus starts to execute thereoxidation voltage decreasing control for decreasing the sensor voltageVss from the increasing end voltage Vup_end (in this embodiment, 0.8 V)with a decreasing rate of the sensor voltage Vss increasing gradually(see the point of time t1 in FIG. 7). When the sensor voltage Vssreaches the decreasing end voltage Vdown_end (in this embodiment, 0.2V), the first embodiment apparatus stops executing the reoxidationvoltage decreasing control (see a point of time t2 in FIG. 7). Thereby,the first embodiment apparatus decreases the sensor voltage Vss from 0.8V to 0.2 V.

In this embodiment, the first embodiment apparatus changes the sensorvoltage Vss in the decomposition voltage increasing control such that aperiod of time from a point of time of starting to increase the sensorvoltage Vss to a point of time of stopping increasing the sensor voltageVss, is 0.1 seconds (=100 ms). In this connection, the period of timefrom the point of time of starting to increase the sensor voltage Vss tothe point of time of stopping increasing the sensor voltage Vss in thedecomposition voltage increasing control of the first embodiment, is notlimited to 0.1 seconds.

Further, in this embodiment, the first embodiment apparatus changes thesensor voltage Vss in the reoxidation voltage decreasing control suchthat a period of time from a point of time of starting to decrease thesensor voltage Vss to a point of time of stopping decreasing the sensorvoltage Vss, is 0.1 seconds (=100 ms). In this connection, the firstembodiment apparatus may be configured to change the sensor voltage Vssin the reoxidation voltage decreasing control such that the period oftime from the point of time of starting to decrease the sensor voltageVss to the point of time of stopping decreasing the sensor voltage Vss,corresponds to a period of time longer than 0.1 seconds and equal to orshorter than 5 seconds.

The first embodiment apparatus acquires the sensor current Iss as thebase current Ibase when the sensor voltage Vss reaches the oxygendecreasing voltage Vox_de (in this embodiment, 0.6 V) while the firstembodiment apparatus executes the reoxidation voltage decreasingcontrol.

Further, the first embodiment apparatus acquires the sensor current Issas a SOx concentration current Iss_sox(n) each time the sensor voltageVss decreases by a predetermined value while the first embodimentapparatus decreases the sensor voltage Vss to the voltage decreasing endvoltage Vdown_end (in this embodiment, 0.2 V) after the sensor voltageVss reaches the oxygen decreasing voltage Vox_de. In addition, the firstembodiment apparatus stores the acquired SOx concentration currentsIss_sox(n) in the RAM in association with the sensor voltage Vss(n) at apoint of time when the first embodiment apparatus acquires each of theSOx concentration currents Iss_sox(n).

The first embodiment apparatus may be configured to acquire the sensorcurrent Iss as the SOx concentration current Iss_sox(1) when the sensorvoltage Vss reaches the oxygen decreasing voltage Vox_de and then,acquire the sensor current Iss as the SOx concentration currentIss_sox(n) each time a predetermined time elapses.

Then, the first embodiment apparatus acquires the integration value S11of the differences dIss(n) between each of the SOx concentrationcurrents iss_sox(n) and the base current Ibase (dIss(n)=Ibase−Iss(n)).

The first embodiment apparatus applies the acquired integration valueS11 to a look-up table Map11Csox(S11) to acquire the exhaust SOxconcentration Csox. The look-up table Map11Csox(S11) is preparedpreviously on the basis of experiments, etc. for determining arelationship between the integration value S11 and the exhaust SOxconcentration in the sensor 10. The exhaust SOx concentration Csoxacquired from the look-up table Map11Csox(S11) increases as theintegration value S11 increases, Hereinafter, the integration value S11will be referred to as “the current difference integration value S11”.

The first embodiment apparatus acquires the exhaust SOx concentrationCsox, using the current difference integration value S11. As describedabove, the current difference integration value S11 is a valuecorrelating with the exhaust SOx concentration. Therefore, the firstembodiment apparatus can acquire the exhaust SOx concentration.

Further, the SOx concentration currents Iss_sox(n) are the sensorcurrents Iss subject to the oxidizing reaction of the sulfur componentproduced by the decomposing reaction of the SOx. In addition, the basecurrent Ibase is the sensor current Iss acquired when the sensor voltageVss reaches the oxygen decreasing voltage Vox_de. Therefore, the basecurrent Ibase is the sensor current Iss subject to the oxidizingreaction of the sulfur component.

Therefore, the current difference integration value S11 is a valueacquired by integrating the current differences dIss(n) which aredifferences between the base current Ibase subject to the oxidizingreaction of the sulfur component and each of the SOx concentrationcurrents Iss_sox(n), respectively. Thus, the current differenceintegration value S11 is a value which includes no or almost nocomponent of the sensor current Iss not subject to the oxidizingreaction of the sulfur component.

Accordingly, (1) the base current Ibase and the SOx concentrationcurrent Iss_sox(n) used for acquiring the current difference integrationvalue S11 are currents subject to the oxidizing reaction of the sulfurcomponent derived from the SOx, (2) the current difference dIss used foracquiring the current difference integration value S11 is a value whichincludes no or almost no component of the sensor current Iss not subjectto the oxidizing reaction of the sulfur component, (3) the currentdifference integration value S11 is a value acquired using the SOxconcentration currents Iss_sox(n).

Therefore, a change of the current difference integration value S11 whenthe exhaust SOx concentration changes, is larger than a change of thedifference between the base current Ibase and the SOx concentrationcurrent Iss_sox when the exhaust SOx concentration changes in case thatthe sensor current not subject to the oxidizing reaction of the sulfurcomponent is used as the base current Ibase. Thus, the currentdifference integration value S11 represents the change of the exhaustSOx concentration definitely. The first embodiment apparatus acquiresthe exhaust SOx concentration Csox, using the current differenceintegration value S11. Thus, the first embodiment apparatus can acquirethe exhaust SOx concentration accurately.

As shown in FIG. 8, the first embodiment apparatus may be configured toincrease the sensor voltage Vss from 0.4 V to 0.8 V in the decompositionvoltage increasing control such that the increasing rate of the sensorvoltage Vss is constant. In addition, as shown in FIG. 8, the firstembodiment apparatus may be configured to decrease the sensor voltageVss from 0.8 V to 0.2 V in the reoxidation voltage decreasing controlsuch that the decreasing rate of the sensor voltage Vss is constant.

Further, the sensor voltage Vss at the point of time of starting toincrease the sensor voltage Vss in the decomposition voltage increasingcontrol, that is, the sensor voltage Vss applied to the sensor cell 15in the constant voltage control, is not limited to 0.4 V. The sensorvoltage Vss at the point of time of starting to increase the sensorvoltage Vss in the decomposition voltage increasing control may be avoltage lower than the oxygen increasing voltage Vox_in. For example,the sensor voltage Vss at the point of time of starting to increase thesensor voltage Vss in the decomposition voltage increasing control, maybe 0.2 V.

Further, the sensor voltage Vss at the point of time of stoppingincreasing the sensor voltage Vss in the decomposition voltageincreasing control, that is, the increasing end voltage Vup_end, is notlimited to 0.8 V. The sensor voltage Vss at the point of time ofstopping increasing the sensor voltage Vss in the decomposition voltageincreasing control, may be a voltage higher than the oxygen increasingvoltage Vox_in.

Further, the sensor voltage Vss at the point of time of stoppingdecreasing the sensor voltage Vss in the reoxidation voltage decreasingcontrol, that is, the voltage decreasing end voltage Vdown_end, is notlimited to 0.2 V. The sensor voltage Vss at the point of time ofstopping decreasing the sensor voltage Vss in the reoxidation voltagedecreasing control, may be a voltage lower than the oxygen decreasingvoltage Vox_de.

Further, the base current Ibase is not limited to the sensor current Isswhen the sensor voltage Vss reaches the oxygen decreasing voltage Vox_de(in this embodiment, 0.6 V) while the first embodiment apparatusexecutes the reoxidation voltage decreasing control. The base currentIbase may be the sensor current Iss when the sensor voltage Vss reachesa voltage equal to or lower than the oxygen decreasing voltage Vox_de.

Further, if an influence of the oxygen included in the exhaust gasreaching the sensor cell 15 A to the sensor current Iss in thereoxidation voltage decreasing control, can be eliminated, the firstembodiment apparatus may be configured to execute the concentrationacquisition voltage control and acquire the exhaust SOx concentrationCsox when the exhaust SOx concentration is requested to be acquiredalthough the engine operation is not in any of the steady operationstate and the idling operation state.

Further, the first embodiment apparatus acquires the exhaust SOxconcentration Csox, using the current difference integration value S11.In this connection, the first embodiment apparatus may be configured toacquire the exhaust SOx concentration Csox, using a value correlatingwith the current difference integration value S11, for example, thecurrent difference integration value S11 corrected by a correctioncoefficient.

Acquisition of Exhaust Oxygen Concentration

As understood referring to FIG. 3, in the sensor 10, there is a limitingcurrent range which is a range of the sensor voltage Vss in which thesensor current Iss is constant, independently of the sensor voltage Vsswhen the exhaust oxygen concentration (i.e., the exhaust gas air-fuelratio A/F) is constant. Therefore, the exhaust oxygen concentration(i.e., the exhaust gas air-fuel ratio A/F) can be acquired by using thesensor current Iss when a voltage within the limiting current range forthe exhaust oxygen concentrations to be acquired, is applied to thesensor cell 15.

As described above, the first embodiment apparatus executes the constantvoltage control for controlling the sensor voltage Vss to 0.4 V when theexhaust SOx concentration is not requested to be acquired. In thisembodiment, the voltage of 0.4 V is the voltage within the limitingcurrent range for the exhaust oxygen concentrations to be acquired.

Accordingly, the first embodiment apparatus acquires the sensor currentIss as an oxygen concentration current Iss_oxy while the firstembodiment apparatus executes the constant voltage control. Then, thefirst embodiment apparatus applies the oxygen concentration currentIss_oxy to a look-up table MapCoxy(Iss_oxy) thereby acquiring theexhaust oxygen concentration Coxy.

The look-up table MapCoxy(Iss_oxy) is prepared previously on the basisof experiments, etc. for determining a relationship between the sensorcurrent Iss and the exhaust oxygen concentration when the sensor voltageVss is controlled to 0.4 V. The exhaust oxygen concentration Coxyacquired from the look-up table MapCoxy(Iss_oxy) increases as the oxygenconcentration current Iss_oxy increases.

Thereby, the first embodiment apparatus can acquire the exhaust oxygenconcentration as well as the exhaust SOx concentration.

It should be noted that the sensor voltage Vss applied in the constantvoltage control is not limited to 0.4 V. The sensor voltage Vss appliedin the constant voltage control may be a voltage in the limiting currentrange for the exhaust oxygen concentrations to be acquired.

Concrete Operation of First Embodiment Apparatus

Next, a concrete operation of the first embodiment apparatus will bedescribed. The CPU of the ECU 90 of the first embodiment apparatus isconfigured or programmed to execute a routine shown in FIG. 9 each timea predetermined time elapses.

Therefore, at a predetermined timing, the CPU starts a process from astep 900 in FIG. 9 and proceeds with the process to a step 905 todetermine whether a value of a SOx concentration acquiring request flagXsox is “1”. The value of the SOx concentration acquiring request flagXsox is set to “1” when the exhaust SOx concentration is requested to beacquired and is set to “0” when the exhaust SOx concentration isacquired.

When the value of the SOx concentration acquiring request flag Xsox is“1”, the CPU determines “Yes” at the step 905 and then, proceeds withthe process to a step 910 to determine whether the engine operation isin the steady operation state or the idling operation state.

When the engine operation is in the steady operation state or the idlingoperation state, the CPU determines “Yes” at the step 910 and then,proceeds with the process to a step 915 to determine whether a value ofa voltage increasing end flag Xup is “0”. The value of the voltageincreasing end flag Xup is set to “1” when the decomposition voltageincreasing control ends and is set to “0” when the reoxidation voltagedecreasing control ends after the decomposition voltage increasingcontrol ends. Immediately after the exhaust SOx concentration isrequested to be acquired, the decomposition voltage increasing controlhas not been executed and thus, the value of the voltage increasing endflag Xup is “0”.

When the value of the voltage increasing end flag Xup is “0” at a timeof executing a process of the step 915, the CPU determines “Yes” at thestep 915 and then, execute a process to a step 920 described below.Then, the CPU proceeds with the process to a step 925.

Step 920: The CPU starts to execute the decomposition voltage increasingcontrol when the CPU has not executed the decomposition voltageincreasing control. On the other hand, the CPU continues to execute thedecomposition voltage increasing control when the CPU already executesthe decomposition voltage increasing control. When the CPU executes theprocess of the step 920 immediately after the CPU first determines “Yes”at the step 915 after the exhaust SOx concentration Csox is requested tobe acquired, the CPU has not executed the decomposition voltageincreasing control. In this case, the CPU starts to execute thedecomposition voltage increasing control. The CPU continues to executethe decomposition voltage increasing control until the CPU determines“Yes” at the step 925.

When the CPU proceeds with the process to the step 925, the CPUdetermines whether the sensor voltage Vss reaches 0.8 V, that is, thesensor voltage Vss is equal to or higher than 0.8 V. When the sensorvoltage Vss is lower than 0.8 V, the CPU determines “No” at the step 925and then, proceeds with the process to a step 995 to terminate thisroutine once.

On the other hand, when the sensor voltage Vss is equal to or higherthan 0.8 V, the CPU determines “Yes” at the step 925 and then, executesprocesses of steps 930 and 935 described below. Then, the CPU proceedswith the process to the step 995 to terminate this routine once.

Step 930: The CPU stops executing the decomposition voltage increasingcontrol.

Step 935: The CPU sets the value of the voltage increasing end flag Xupto “1”. Thereby, when the CPU proceeds with the process to the step 915,the CPU determines “No” at the step 915.

When the value of the voltage increasing end flag Xup is “1” at a timeof executing a process of the step 915, the CPU determines “No” at thestep 915 and then, executes a process of a step 940 described below.Then, the CPU proceeds with the process to a step 942.

Step 940: The CPU starts to execute the reoxidation voltage decreasingcontrol when the CPU has not executed the reoxidation voltage decreasingcontrol. On the other hand, the CPU continues to execute the reoxidationvoltage decreasing control when the CPU already executes the reoxidationvoltage decreasing control. When the CPU executes the process of thestep 940 immediately after the CPU first determines “No” at the step 915after the exhaust SOx concentration Csox is requested o be acquired, theCPU has not executed the reoxidation voltage decreasing control. In thiscase, the CPU starts to execute the reoxidation voltage decreasingcontrol. The CPU continues to execute the reoxidation voltage decreasingcontrol until the CPU determines “Yes” at a step 950.

When the CPU proceeds with the process to the step 942, the CPUdetermines whether the sensor voltage Vss is equal to or lower than 0.6V, that is, the sensor voltage Vss is equal to or lower than the oxygendecreasing voltage Vox_de. When the sensor voltage Vss is higher than0.6 V, the CPU determines “No” at the step 942 and then, proceeds withthe process to the step 995 to terminate this routine once.

On the other hand, when the sensor voltage Vss is equal to or lower than0.6 V, the CPU determines “Yes” at the step 942 and then, executes aprocess of a step 945 described below. Then, the CPU proceeds with theprocess to the step 950.

Step 945: The CPU acquires the sensor current Iss and stores theacquired sensor current Iss as the SOx concentration current Iss_sox(n)in the RAM in association with the sensor voltage Vss corresponding tothe acquisition of the sensor current Iss.

When the CPU proceeds with the process to the step 950, the CPUdetermines whether the sensor voltage Vss reaches 0.2 V, that is, thesensor voltage Vss is equal to or lower than 0.2 V. When the sensorvoltage Vss is higher than 0.2 V, the CPU determines “No” at the step950 and then, proceeds with the process to the step 995 to terminatethis routine once.

On the other hand, when the sensor voltage Vss is equal to or lower than0.2 V, the CPU determines “Yes” at the step 950 and then, executesprocesses of steps 955 to 975 described below. Then, the CPU proceedswith the process to the step 995 to terminate this routine once.

Step 955: The CPU stops executing the reoxidation voltage decreasingcontrol.

Step 960: The CPU executes a routine shown by a flowchart in FIG. 10.

Therefore, when the CPU proceeds with the process to the step 960, theCPU starts a process from a step 1000 in FIG. 10 and then, executesprocesses of steps 1010 to 1040 described below. Then, the CPU proceedswith the process to the step 975 in FIG. 9 via a step 1095.

Step 1010: The CPU acquires the SOx concentration current Iss_sox(1) asthe base current Ibase from the SOx concentration currents Iss_sox(n).The SOx concentration current Iss_sox(1) is the sensor current acquiredwhen the sensor voltage Vss reaches 0.6 V, that is, the oxygendecreasing voltage Vox_de.

Step 1020: The CPU acquires the difference between the base currentIbase and each of the SOx concentration currents Iss_sox(n) as thecurrent difference dIss(n) (=Ibase−Iss_sox(n)).

Step 1030: The CPU acquires the integration value of the currentdifferences dIss(n) as the current difference integration value S11(=Σ(dIss(n))).

Step 1040: The CPU applies the current difference integration value S11to the look-up table Map11Csox(S11) to acquire the exhaust SOxconcentration Csox.

When the CPU proceeds with the process to the step 975 in FIG. 9, theCPU sets the values of the SOx concentration acquisition request flagXsox and the voltage increasing end flag Xup to “0”, respectively.

When the value of the SOx concentration acquiring request flag Xsox is“0” at a time of executing a process of the step 905 in FIG. 9, and theengine operation is not in any of the steady operation state and theidling operation state at a time of executing a process of the step 910in FIG. 9, the CPU determines “No” at any of the steps 905 and 910 andthen, executes processes of steps 980 to 990. Then, CPU proceeds withthe process to the step 995 to terminate this routine once.

Step 980: The CPU starts to execute the constant voltage control forcontrolling the sensor voltage Vss to 0.4 V when the CPU has notexecuted the constant voltage control. On the other hand, the CPUcontinues to execute the constant voltage control when the CPU alreadyexecutes the constant voltage control.

Step 985: The CPU acquires the sensor current Iss and stores theacquired sensor current Iss as the oxygen concentration current Iss_oxyin the RAM.

Step 990: The CPU applies the oxygen concentration current Iss_oxy tothe look-up table MapCoxy(Iss_oxy) to acquire the exhaust oxygenconcentration Coxy.

The first embodiment apparatus can acquire the exhaust SOx concentrationand the exhaust oxygen concentration by executing the routine shown inFIG. 9.

Further, when the exhaust SOx concentration is equal to or lower than anupper limit concentration Csox_limit designated by law but is near theupper limit concentration Csox_limit, it is desired to determine thatthe exhaust SOx concentration is near the upper limit concentrationCsox_limit in order to inform that the exhaust SOx concentration is nearthe upper limit concentration Csox_limit.

Accordingly, the CPU of the ECU 90 of the first embodiment apparatus isconfigured or programmed to execute a routine shown by a flowchart inFIG. 11 each time a predetermined time elapses. Therefore, at apredetermined timing, the CPU starts a process from a step 1100 in FIG.11 and proceeds with the process to a step 1110 to determine whether theexhaust SOx concentration Csox acquired at the step 1040 in FIG. 10 islarger than an upper limit concentration Cth. The upper limitconcentration Cth is a permissible upper limit value of the exhaust SOxconcentration.

When the exhaust SOx concentration Csox is larger than the upper limitconcentration Cth, the CPU determines “Yes” at the step 1110 and then,proceeds with the process to a step 1120 to determine that the exhaustSOx concentration is larger than the upper limit concentration Cth.Then, the CPU proceeds with the process to a step 1195 to terminate thisroutine once.

On the other hand, when the exhaust SOx concentration Csox is equal toor smaller than the upper limit concentration Cth, the CPU determines“No” at the step 1110 and then, proceeds with the process to a step 1130to determine that the exhaust SOx concentration is equal to or smallerthan the upper limit concentration Cth. Then, the CPU proceeds with theprocess to the step 1195 to terminate this routine once.

The first embodiment apparatus can determine whether the exhaust SOxconcentration is larger than the upper limit concentration by executingthe routine shown in FIG. 11.

Modified Example of First Embodiment

Next, the SOx concentration acquiring apparatus of the internalcombustion engine according to a modified example of the firstembodiment will be described. Hereinafter, the SOx concentrationacquiring apparatus according to the modified example of the firstembodiment will be referred to as “the first modified apparatus”.

Summary of Operation of First Modified Apparatus

As described above, the first embodiment apparatus uses the currentdifference dIss(n) in which the component of the sensor current Iss notsubject to the oxidizing reaction of the surfur component is eliminated.Thus, the first embodiment apparatus can acquire the exhaust SOxconcentration accurately.

In this regard, an average change rate Rave of the sensor current Isschanging from a high voltage current Ihigh which is the sensor currentIss at the oxygen decreasing voltage Vox_de (in this embodiment, 0.6 V)to a low voltage current Ilow which is the sensor current Iss at thevoltage decreasing end voltage Vdown_end (in this embodiment, 0.2 V), isgenerally equal to an average change rate of the sensor current Isschanging while the sensor voltage Vss decreases from the oxygendecreasing voltage Vox_de to the voltage decreasing end voltageVdown_end when the exhaust SOx concentration is zero. In thisembodiment, the low voltage current Ilow is 0.4 mA, and the high voltagecurrent Ihigh is 0.42 mA.

Therefore, when currents at the sensor voltages Vss(n), at which the SOxconcentration currents iss_sox(n) are acquired, are acquired as the basecurrents Ibase(n) from currents changing from the high-voltage currentIhigh at the average change rate Rave, that is, currents on a line shownby a reference sign Lbase in FIG. 12, and differences between each ofthe base currents Ibase(n) at the sensor voltages Vss(n), at which theSOx concentration currents Iss_sox(n), and each of the SOx concentrationcurrents Iss_sox(n) are acquired as current differences dIss(n), each ofthe current differences dIss(n) is a value in which component of thesensor current Iss not subject to the oxidizing reaction of the sulfurcomponent is eliminated to a large extent, compared with the currentdifferences dIss(n) acquired by the first embodiment apparatus.

Thus, the exhaust SOx concentration Csox can be acquired more accuratelyby acquiring the exhaust SOx concentration Csox on the basis of theintegration value S12 of the current differences dIss(n). In this case,the integration value S12 corresponds to an area shown by a referencesign A12 in FIG. 12.

Accordingly, the first modified apparatus executes a preliminary voltagecontrol including a preliminary voltage increasing control and apreliminary voltage decreasing control when the exhaust SOxconcentration is requested to be acquired, and the engine operation isin any of the steady operation state and the idling operation state.

When the first modified apparatus starts to execute the preliminaryvoltage control, the first modified apparatus executes a voltageincreasing preparation control for decreasing the sensor voltage Vssfrom 0.4 V. In this regard, the first modified apparatus acquires thesensor current Iss as the reference current Iref (in this embodiment,0.5 mA) when the sensor voltage Vss is controlled to 0.4 V. Then, thefirst embodiment apparatus stops executing the voltage increasingpreparation control when the sensor voltage Vss reaches 0.2 V.

When the first modified apparatus stops executing the voltage increasingpreparation control, the first modified apparatus executes thepreliminary voltage increasing control for increasing the sensor voltageVss from 0.2 V. The first modified apparatus stops executing thepreliminary voltage increasing control when the sensor voltage Vssreaches 0.3 V. The sensor current Iss changes as shown by a line LU inFIG. 13 while the preliminary voltage increasing control is executed.

When the first modified apparatus stops executing the preliminaryvoltage increasing control, the first modified apparatus executes thepreliminary voltage decreasing control for decreasing the sensor voltageVss from 0.3 V. When the sensor voltage Vss reaches 0.2 V, the firstmodified apparatus stops executing the preliminary voltage decreasingcontrol. Thus, the first modified apparatus stops executing thepreliminary voltage control. The sensor current Iss changes as shown bya line LD in FIG. 13 while the first modified apparatus executes thepreliminary voltage decreasing control. The first modified apparatusacquires the sensor current Iss as the low-voltage current Ilow when thefirst modified apparatus stops executing the preliminary voltagedecreasing control, that is, the sensor voltage Vss reaches 0.2 V.

When the first modified apparatus stops executing the preliminaryvoltage control, the first modified apparatus executes the concentrationacquisition voltage control. When the first embodiment apparatus startsto execute the concentration acquisition voltage control, the firstmodified apparatus executes the decomposition voltage increasing controlfor increasing the sensor voltage Vss from 0.2 V. When the sensorvoltage Vss reaches 0.8 V, the first modified apparatus stops executingthe decomposition voltage increasing control.

When the first modified apparatus stops executing the decompositionvoltage increasing control, the first modified apparatus executes thereoxidation voltage decreasing control for decreasing the sensor voltageVss from 0.8 V. When the sensor voltage Vss reaches 0.2 V, the firstmodified apparatus stops executing the reoxidation voltage decreasingcontrol. The first modified apparatus acquires the sensor current Iss asthe SOx concentration current Iss_sox(n) each time the sensor voltageVss decreases by a predetermined value while the sensor voltage Vssdecreases from 0.6 V to 0.2 V. The first modified apparatus stores theacquired SOx concentration currents Iss_sox(n) in the RAM in associationwith the sensor voltages Vss(n), at which the SOx concentration currentsIss_sox(n) are acquired.

The first modified apparatus acquires an integration value ofdifferences between the reference current Iref and each of the SOxconcentration currents Iss_sox(n) as a first integration value S121(=Σ(Iref−Iss_sox(n))). The first integration value S121 corresponds toan area shown by a reference sign A121 in FIG. 14A.

Further, the first modified apparatus acquires the sensor current Iss asthe high-voltage current Ihigh when the sensor voltage Vss reaches theoxygen decreasing voltage Vox_de (in this embodiment, 0.6 V) while thefirst modified apparatus executes the reoxidation voltage decreasingcontrol. The first modified apparatus acquires an average change rate ofthe sensor current Iss as a base current change rate R while the sensorcurrent Iss decreases from the high-voltage current Ihigh to thelow-voltage current Ilow. The first modified apparatus acquires currentsat the sensor voltages Vss(n), at which the SOx concentration currentsIss_sox(n) are acquired, as base currents Ibase(n) from the currentschanging from the high-voltage current Ihigh at the base current changerate R.

The first modified apparatus acquires an integration value ofdifferences between the reference current Iref and each of the basecurrents Ibase(n) as a second integration value S122(=Σ(Iref−Ibase(n))). The second integration value S122 corresponds to anarea shown by a reference sign A122 in FIG. 14B.

The first modified apparatus subtracts the second integration value S122from the first integration value S121, thereby acquiring a currentdifference integration value S12 (=S121−S122).

In particular, the first modified apparatus acquires differences betweeneach of the base currents Ibase(n) and each of the SOx concentrationcurrents Iss_sox(n) as the current differences dIss(n) by the methoddescribed above. The first modified apparatus acquires the integrationvalue of the current differences dIss(n) as the current differenceintegration value S12.

The first modified apparatus applies the current difference integrationvalue S12 to a look-up table Map12Csox(S12), thereby acquiring theexhaust SOx concentration Csox. The look-up table Map12Csox(S12) isprepared previously on the basis of experiments, etc. for determining arelationship between the current difference integration value S12 andthe exhaust SOx concentration in the sensor 10. The exhaust SOxconcentration Csox acquired from the look-up table Map12Csox(S12)increases as the current difference integration value S12 increases.

The first modified apparatus acquires the exhaust SOx concentrationCsox, using the current difference integration value S12. The currentdifference integration value S12 is a value correlating with the exhaustSOx concentration. Therefore, the first modified apparatus can acquirethe exhaust SOx concentration.

Further, the base current change rate R is a value near the change rateof the sensor current Iss changing after the sensor voltage Vss reachesthe oxygen decreasing voltage Vox_de while the exhaust SOx concentrationis zero. Therefore, the differences between each of the base currentsIbase(n) and each of the SOx concentration currents Iss_sox(n) are valuein which components of the sensor current Iss not subject to theoxidizing reaction of the sulfur component are eliminated since thecurrents acquired on the basis of the base current change rate R is usedas the base currents Ibase(n). Thus, the change rate of the currentdifference integration value S12 is larger than the change rate of thedifference between the base current and the SOx concentration currentIss_sox acquired, using the current not subject to the oxidizingreaction of the sulfur component as the base current when the exhaustSOx concentration changes. Therefore, the current difference integrationvalue S12 represents the change of the exhaust SOx concentration. Thus,the exhaust SOx concentration can be acquired accurately.

In the first modified example, the low-voltage current Ilow is lowerthan the high-voltage current Ihigh. In this regard, the first modifiedapparatus may be applied to a liminting current sensor which thelow-voltage current Ilow is larger than the high-voltage current Ihigh.

Further, when a large calculation load of the CPU of the ECU 90 of thefirst modified apparatus can be permitted, or the CPU has suffficientlarge calculation ability, the first modified apparatus may beconfigured to acquire the differences dIss(n) between each of the basecurrents Ibase(n) and each of the SOx concentration current Iss_sox(n)directly and integrate the differences dIss(n) to acquire the currentdifference integration value S12.

Further, the first modified apparatus executes the preliminary voltagecontrol before the first modified apparatus executes the concentrationacquisition voltage control. In this regard, the first modifiedapparatus may be configured to execute the preliminary voltage controlafter the first modified apparatus executes the concentrationacquisition voltage control. However, in this case, the first sensorelectrode 15A may be subject to the decomposing reaction of the SOx andthe oxidizing reaction of the sulfur component occuring during theconcentration acquisition voltage control when the first modifiedapparatus executes the preliminary voltage control. Therefore,preferably, the first modified apparatus executes the preliminaryvoltage control before the first modified apparatus executes theconcentration acquisition voltage control.

Furthermore, the first modified apparatus acquires the sensor currentIss as the low-voltage current Ilow when the sensor voltage Vss reachesthe voltage decreasing end voltage Vdown_end (in this example, 0.2 V).In this regard, the first modified apparatus may be configured toacquire the sensor current Iss as the low-voltage current Ilow when thesensor voltage Vss reaches a voltage other than the voltage decreasingend voltage Vdown_end. For example, the first modified apparatus may beconfigured to acquire the sensor current Iss as the low-voltage currentIlow when the sensor voltage Vss reaches a voltage lower than the oxygendecreasing voltage Vox_de.

Further, the first modified apparatus aquires the sensor current Iss asthe high-voltage current Ihigh when the sensor voltage Vss reaches theoxygen decreasing voltage Vox_de (in this example, 0.6 V). In thisregard, the first modified apparatus may be configured to acquire thesensor current Iss as the high-voltage current Ihigh when the sensorvoltage Vss reaches a voltage other than the oxygen decreasing voltageVox_de. For example, the first modified apparatus may be configured toacquire the sensor current Iss as the high-voltage current Ihigh whenthe sensor voltage Vss reaches a voltage higher than the sensor voltageVss, at which the low-voltage current Ilow is acquired, and equal to orlower than the oxygen decreasing voltage Vox_de.

Furthermore, the first modified apparatus acquires the sensor currentIss as the low-voltage current Ilow when the sensor voltage Vss reaches0.2 V while the first modified apparatus executes the preliminaryvoltage decreasing control. In this regard, the first modified apparatusmay be configured to acquire the sensor current Iss as the low-voltagecurrent Ilow when the sensor voltage Vss reaches a voltage lower thanthe oxygen decreasing voltage Vox_de, in particular, the sensor voltageVss reaches 0.2 V while the first modified apparatus executes thereoxidation voltage decreasing control without executing the preliminaryvoltage control.

Further, the first modified apparatus acquires the exhaust SOxconcentration Csox, using the current difference integration value S12.In this regard, the first modified apparatus may be configured toacquire the exhaust SOx concentration Csox, using a value correlatingwith the current difference integration value S12, for example, using avalue acquired by correcting the current difference integration valueS12 by a correction coefficient.

Concrete Operation of First Modified Apparatus

Next, a concrete operation of the first modified apparatus will bedescribed. The CPU of the ECU 90 of the first modified apparatus isconfigured or programmed to execute a routine shown in FIG. 15 each timea predetermined time elapses. Therefore, at a predetermined timing, theCPU starts a process from a step 1500 in FIG. 15 and proceeds with theprocess to a step 1510 to determine whether a value of a SOxconcentration acquiring request flag Xsox is “1”.

When the value of the SOx concentration acquiring request flag Xsox is“1”, the CPU determines “Yes” at the step 1510 and then, proceeds withthe process to a step 1515 to determine whether the engine operation isin the steady operation state or the idling operation state.

When the engine operation is in the steady operation state or the idlingoperation state, the CPU determines “Yes” at the step 1515 and then,proceeds with the process to a step 1520 to determine whether a value ofa preliminary voltage control end flag Xalt is “0”. The value of thepreliminary voltage control end flag Xalt is set to “1” when thepreliminary voltage control ends and is set to “0” when theconcentration acquisition voltage control ends after the preliminaryvoltage control ends. Immediately after the exhaust SOx concentration isrequested to be acquired, the preliminary voltage control has not beenexecuted and thus, the value of the preliminary voltage control end flagXalt is “0”.

When the value of the preliminary voltage control end flag Xalt is “0”at a time of executing a process of the step 1520, the CPU determines“Yes” at the step 1520 and then, proceeds with the process to a step1530 to execute a routine shown by a flowchart in FIG. 16.

Therefore, when the CPU proceeds with the process to the step 1530 inFIG. 15, the CPU starts a process from a step 1600 in FIG. 16 and then,proceeds with the process to a step 1605 to determine whether a value ofa preparation end flag Xpre is “0”. The value of the preparation endflag Xpre is set to “1” when the voltage increasing preparation controlends and is set to “0” when the preliminary voltage decreasing controlends after the voltage increasing preparation control ends.

When the value of the preparation end flag Xpre is “0” at a time ofexecuting a process of the step 1605, the CPU determines “Yes” at thestep 1605 and then, executes a process of a step 1610 described below.Then, the CPU proceeds with the process to a step 1615.

Step 1610: The CPU starts to execute the voltage increasing preparationcontrol when the CPU has not executed the voltage increasing preparationcontrol. On the other hand, the CPU continues to execute the voltageincreasing preparation control when the CPU already executes the voltageincreasing preparation control. When the CPU executes the process of thestep 1610 immediately after the CPU first determines “Yes” at the step1605, the CPU has not executed the voltage increasing preparationcontrol. In this case, the CPU starts to execute the voltage increasingpreparation control. The CPU continues to execute the voltage increasingpreparation control until the CPU determines “Yes” at the step 1615.

When the CPU proceeds with the process to the step 1615, the CPUdetermines whether the sensor voltage Vss reaches 0.2 V, that is, thesensor voltage Vss is equal to or lower than 0.2 V. When the sensorvoltage Vss is higher than 0.2 V, the CPU determines “No” at the step1615 and then, proceeds with the process to a step 1595 in FIG. 15 via astep 1695 to terminate this routine once.

On the other hand, when the sensor voltage Vss is equal to or lower than0.2 V, the CPU determines “Yes” at the step 1615 and then, executesprocesses of steps 1620 and 1625 described below. Then, the CPU proceedswith the process to the step 1595 in FIG. 15 via the step 1695 toterminate this routine once.

Step 1620: The CPU stops executing the voltage increasing preparationcontrol.

Step 1625: The CPU sets the value of the preparation end flag Xpre to“1”. Thereby, when the CPU proceeds with the process to the step 1605,the CPU determines “No” at the step 1605.

When the value of the preparation end flag Xpre is “1” at a time ofexecuting a process of the step 1605, the CPU determines “No” at thestep 1605 and then, proceeds with the process to a step 1630 to deteminewhether a value of a voltage increasing end flag Xup1 is “0”. The valueof the voltage increasing end flag Xup1 is set to “1” when thepreliminary voltage increasing control ends and is set to “0” when thepreliminary voltage decreasing control ends after the preliminaryvoltage increasing control ends.

When the value of the voltage increasing end flag Xup1 is “0” at a timeof executing a process of the step 1630, the CPU determines “Yes” at thestep 1630 and then, execute a process to a step 1635 described below.Then, the CPU proceeds with the process to a step 1640.

Step 1635: The CPU starts to execute the preliminary voltage increasingcontrol when the CPU has not executed the preliminary voltage increasingcontrol. On the other hand, the CPU continues to execute the preliminaryvoltage increasing control when the CPU already executes the preliminaryvoltage increasing control. When the CPU executes the process of thestep 1635 immediately after the CPU first determines “Yes” at the step1630, the CPU has not executed the preliminary voltage increasingcontrol. In this case, the CPU starts to execute the preliminary voltageincreasing control. The CPU continues to execute the preliminary voltageincreasing control until the CPU determines “Yes” at the step 1640.

When the CPU proceeds with the process to the step 1640, the CPUdetermines whether the sensor voltage Vss reaches 0.3 V, that is, thesensor voltage Vss is equal to or higher than 0.3 V. When the sensorvoltage Vss is lower than 0.3 V, the CPU determines “No” at the step1640 and then, proceeds with the process to the step 1595 in FIG. 15 viathe step 1695 to terminate this routine once.

On the other hand, when the sensor voltage Vss is equal to or higherthan 0.3 V, the CPU determines “Yes” at the step 1640 and then, executesprocesses of steps 1645 and 1650 described below. Then, the CPU proceedswith the process to the step 1595 in FIG. 15 via the step 1695 toterminate this routine once.

Step 1645: The CPU stops executing the preparation voltage increasingcontrol.

Step 1650: The CPU sets the value of the voltage increasing end flagXup1 to “1”. Thereby, when the CPU proceeds with the process to the step1630, the CPU determines “No” at the step 1630.

When the value of the voltage increasing end flag Xup1 is “1” at a timeof executing a process of the step 1630, the CPU determines “No” at thestep 1630 and then, executes a process of a step 1655 described below.Then, the CPU proceeds with the process to a step 1660.

Step 1655: The CPU starts to execute the preliminary voltage decreasingcontrol when the CPU has not executed the preliminary voltage decreasingcontrol. On the other hand, the CPU continues to execute the preliminaryvoltage decreasing control when the CPU already executes the preliminaryvoltage decreasing control. When the CPU executes the process of thestep 1655 immediately after the CPU first determines “No” at the step1630, the CPU has not executed the preliminary voltage decreasingcontrol. In this case, the CPU starts to execute the preliminary voltagedecreasing control. The CPU continues to execute the preliminary voltagedecreasing control until the CPU determines “Yes” at the step 1660.

When the CPU proceeds with the process to the step 1660, the CPUdetermines whether the sensor voltage Vss reaches 0.2 V, that is, thesensor voltage Vss is equal to or lower than 0.2 V. When the sensorvoltage Vss is higher than 0.2 V, the CPU determines “No” at the step1660 and then, proceeds with the process to the step 1555 in FIG. 15 viathe step 1695 to terminate this routine once.

On the other hand, when the sensor voltage Vss is equal to or lower than0.2 V, the CPU determines “Yes” at the step 1660 and then, executesprocesses of steps 1665 to 1675 described below. Then, the CPU proceedswith the process to the step 1595 in FIG. 15 via the step 1695 toterminate this routine once.

Step 1665: The CPU stops executing the preliminary voltage decreasingcontrol.

Step 1670: The CPU acquires the SOx concentration current Iss and storesthe acquired SOx concentration current Iss as the low-voltage currentIlow in the RAM

Step 1675: The CPU sets the value of the preliminary voltage control endflag Xalt to “1”. Thereby, when the CPU proceeds with the process to thestep 1520 in FIG. 15, the CPU determines “No” at the step 1520. Further,the CPU sets the values of the preparation end flag Xpre and the voltageincreasing end flag Xup1 to “0”, respectively.

When the value of the preliminary voltage control end flag Xalt is “1”at a time of executing a process of the step 1520 in FIG. 15, the CPUdetermines “No” at the step 1520 and then, proceeds with the process toa step 1540 to execute a routine shown by a flowchart in FIG. 17.

Therefore, when the CPU proceeds with the process to the step 1540, theCPU starts a process from a step 1700 in FIG. 17 and then, proceeds withthe process to a step 1705 to determine whether a value of a voltageincreasing end flag Xup2 is “0”. The value of the voltage increasing endflag Xup2 is set to “1” when the decomposition voltage increasingcontrol ends and is set to “0” when the reoxidation voltage decreasingcontrol ends after the decomposition voltage increasing control ends.

When the value of the voltage increasing end flag Xup2 is “0” at a timeof executing a process of the step 1705, the CPU determines “Yes” at thestep 1705 and then, execute a process to a step 1710 described below.Then, the CPU proceeds with the process to a step 1715.

Step 1710: The CPU starts to execute the decomposition voltageincreasing control when the CPU has not executed the decompositionvoltage increasing control. On the other hand, the CPU continues toexecute the decomposition voltage increasing control when the CPUalready executes the decomposition voltage increasing control. When theCPU executes the process of the step 1710 immediately after the CPUfirst determines “Yes” at the step 1705, the CPU has not executed thedecomposition voltage increasing control. In this case, the CPU startsto execute the decomposition voltage increasing control. The CPUcontinues to execute the decomposition voltage increasing control untilthe CPU determines “Yes” at the step 1715.

When the CPU proceeds with the process to the step 1715, the CPUdetermines whether the sensor voltage Vss reaches 0.8 V, that is, thesensor voltage Vss is equal to or higher than 0.8 V. When the sensorvoltage Vss is lower than 0.8 V, the CPU determines “No” at the step1715 and then, proceeds with the process to the step 1595 in FIG. 15 viaa step 1795 to terminate this routine once.

On the other hand, when the sensor voltage Vss is equal to or higherthan 0.8 V, the CPU determines “Yes” at the step 1715 and then, executesprocesses of steps 1720 and 1725 described below. Then, the CPU proceedswith the process to the step 1595 in FIG. 15 via the step 1795 toterminate this routine once.

Step 1720: The CPU stops executing the decomposition voltage increasingcontrol.

Step 1725: The CPU sets the value of the voltage increasing end flagXup2 to “1”. Thereby, when the CPU proceeds with the process to the step1705, the CPU determines “No” at the step 1705.

When the value of the voltage increasing end flag Xup2 is “1” at a timeof executing a process of the step 1705, the CPU determines “No” at thestep 1705 and then, executes a process of a step 1730 described below.Then, the CPU proceeds with the process to a step 1732.

Step 1730: The CPU starts to execute the reoxidation voltage decreasingcontrol when the CPU has not executed the reoxidation voltage decreasingcontrol. On the other hand, the CPU continues to execute the reoxidationvoltage decreasing control when the CPU already executes the reoxidationvoltage decreasing control. When the CPU executes the process of thestep 1730 immediately after the CPU first determines “No” at the step1705, the CPU has not executed the reoxidation voltage decreasingcontrol. In this case, the CPU starts to execute the reoxidation voltagedecreasing control. The CPU continues to execute the reoxidation voltagedecreasing control until the CPU determines “Yes” at the step 1740.

When the CPU proceeds with the process to the step 1732, the CPUdetermines whether the sensor voltage Vss is equal to or lower than 0.6V. When the sensor voltage Vss is higher than 0.6 V, the CPU determines“No” at the step 1732 and then, proceeds with the process to the step1595 in FIG. 15 via the step 1795 to terminate this routine once.

On the other hand, when the sensor voltage Vss is equal to or lower than0.6 V, the CPU determines “Yes” at the step 1732 and then, executes aprocess of a step 1735 described below. Then, the CPU proceeds with theprocess to a step 1740.

Step 1735: The CPU acquires the sensor current Iss and stores theacquired sensor current Iss as the SOx concentration current Iss_sox(n)in the RAM in association with the sensor voltage Vss corresponding tothe acquisition of the sensor current Iss.

When the CPU proceeds with the process to the step 1740, the CPUdetermines whether the sensor voltage Vss reaches 0.2 V, that is, thesensor voltage Vss is equal to or lower than 0.2 V. When the sensorvoltage Vss is higher than 0.2 V, the CPU determines “No” at the step1740 and then, proceeds with the process to the step 1595 in FIG. 15 viathe step 1795 to terminate this routine once.

On the other hand, when the sensor voltage Vss is equal to or lower than0.2 V, the CPU determines “Yes” at the step 1740 and then, executesprocesses of steps 1745 to 1775 described below. Then, the CPU proceedswith the process to the step 1595 in FIG. 15 via the step 1795 toterminate this routine once.

Step 1745: The CPU stops executing the reoxidation voltage decreasingcontrol.

Step 1750: The CPU executes a routine shown by a flowchart in FIG. 18.

Therefore, when the CPU proceeds with the process to the step 1750, theCPU starts a process from a step 1800 in FIG. 18 and then, executesprocesses of steps 1805 to 1840 described below.

Step 1805: The CPU acquires, as the reference current Iref, the sensorcurrent Iss acquired and stored in the RAM at a step 1560 decscribedlater immediately before the CPU determines “Yes” at the step 1515 inFIG. 15 after the exhaust SOx concentration Csox is requested to beacquired.

Step 1810: The CPU acquires the SOx concentration current Iss_sox(1)from the SOx concentration currents Iss_sox(n) as the high-voltagecurrent Ihigh. The SOx concentration current Iss_sox(1) is the sensorcurrent acquired when the sensor voltage Vss reaches 0.6 V, that is, theoxygen decreasing voltage Vox_de.

Step 1815: The CPU acquires, as the base current change rate R, thechange rate of the sensor current Iss changing while the sensor currentIss changes from the high-voltage current Ihigh to the low-voltagecurrent Ilow.

Step 1820: The CPU acquires, as the base currents Ibase(n), the currentsat the sensor voltages Vss(n), at which the SOx concentration currentsIss_sox(n), from the currents changing from the high-voltage currentIhigh at the base current change rate R.

Step 1825: The CPU acquires, as the first integration value S121(=Σ(Iref−Iss_sox(n))), the integration value of the differences betweenthe reference current Iref and each of the SOx concentration currentsIss_sox(n).

Step 1830: The CPU acquires, as the second integration value S122(=Σ(Iref−Ibase(n)), the integration value of the differences between thereference current Iref and each of the base currents Ibase(n).

Step 1835: The CPU subtracts the second integration value S122 from thefirst integration value S121 to acquire the current differenceintegration value S12 (=S121−S122).

Step 1840: The CPU applies the current difference integration value S12to the look-up table Map12Csox(S12) to acquire the exhaust SOxconcentration Csox.

Step 1775 in FIG. 17: The CPU sets the SOx concentration acquisitionrequest flag Xsox, the preliminary voltage control end flag Xalt, andthe voltage increasing end flag Xup2 to “0”, respectively.

When the value of the SOx concentration acquiring request flag Xsox is“0” at a time of executing a process of the step 1510 in FIG. 15, andthe engine operation is not in any of the steady operation state and theidling operation state at a time of executing a process of the step 1515in FIG. 15, the CPU determines “No” at any of the steps 1510 and 1515and then, executes processes of steps 1550 to 1570 described below.Then, CPU proceeds with the process to the step 1595 to terminate thisroutine once.

Step 1550: The CPU starts to execute the constant voltage control forcontrolling the sensor voltage Vss to 0.4 V when the CPU has notexecuted the constant voltage control. On the other hand, the CPUcontinues to execute the constant voltage control when the CPU alreadyexecutes the constant voltage control.

Step 1560: The CPU acquires the sensor current Iss and stores theacquired sensor current Iss as the oxygen concentration current Iss_oxyin the RAM.

Step 1570: The CPU applies the oxygen concentration current Iss_oxy tothe look-up table MapCoxy(Iss_oxy) to acquire the exhaust oxygenconcentration Coxy.

The first modified apparatus can acquire the exhaust SOx concentrationand the exhaust oxygen concentration by executing the routines shown inFIGS. 15 to 17.

Further, the CPU of the ECU 90 of the first modified apparatus isconfigured or programmed to execute the routine shown in FIG. 11 eachtime a predetermined time elapses. When the CPU of the first modifiedapparatus executes the routine shown in FIG. 11, the CPU determineswhether the exhaust SOx concentration Csox acquired at the step 1840 inFIG. 18 is larger than the upper limit concentration Cth at the step1110.

The first modified apparatus can determine whether the exhaust SOxconcentration is larger than the upper limit concentration by executingthe routine shown in FIG. 11.

It should be noted that the first modified apparatus may be configuredto acquire, as the low-voltage current Ilow, the sensor current Iss whenthe sensor voltage Vss reaches 0.2 V while the first modified apparatusexecutes the reoxidation voltage decreasing control. In this case, theCPU of the ECU 90 of the first modified apparatus is configured orprogrammed to execute a routine by a flowchart shown in FIG. 19 eachtime a predetermined time elapses.

Therefore, at a predetermined timing, the CPU of the first modifiedapparatus starts a process from a step 1900 in FIG. 19. Processes ofsteps 1910, 1915, and 1950 to 1970 in FIG. 19 are the same as theprocesses of the steps 1510, 1515, and 1550 to 1570 in FIG. 15,respectively.

When the CPU determines “Yes” at the step 1915, the CPU proceeds withthe process to a step 1930 to execute the routine shown in FIG. 17. Inthis case, the process of setting the value of the preliminary voltagecontrol end flag Xalt to “0” is omitted at the step 1775 in FIG. 17.

Further, when the CPU of the first modified apparatus executes theroutine shown in FIG. 18 at the step 1750 in FIG. 17, the CPU of thefirst modified apparatus executes a process of a step 2080 shown in FIG.20 in place of the process of the step 1810. The CPU of the firstmodified apparatus acquires the SOx concentration current Iss_sox(1),which is acquired when the sensor voltage Vss reaches 0.6 V (i.e., theoxygen decreasing voltage Vox_de), from the SOx concentration currentsIss_sox(n) as the high-voltage current Ihigh at the step 2080. Inaddition, the CPU of the first modified apparatus acquires the SOxconcentration current Iss_sox(n), which is acquired when the sensorvoltage Vss reaches 0.2 V (i.e., the voltage decreasing end voltageVdown_end), from the SOx concentration currents Iss_sox(n) as thelow-voltage current Ilow at the step 2080.

Second Embodiment

Next, the SOx concentration acquiring apparatus of the internalcombustion engine according to a second embodiment of the invention willbe described. The SOx concentration acquiring apparatus according to thesecond embodiment of the invention is applied to the internal combustionengine 50 shown in FIG. 21. The internal combustion engine 50 shown inFIG. 21 is the same as the internal combustion engine 50 shown inFIG. 1. Hereinafter, the SOx concentration acquiring apparatus accordingto the second embodiment will be referred to as “the second embodimentapparatus”.

The second embodiment apparatus includes a limiting current sensor 20having an inner configuration shown in FIG. 22, a pump cell voltagesource 25C, a sensor cell voltage source 26C, a pump cell ammeter 25D, asensor cell ammeter 26D, a sensor cell voltmeter 26E, and the ECU 90.The sensor 20 is a two-cell type limiting current sensor. The sensor 20is provided on the exhaust pipe 83.

As shown in FIG. 22, the sensor 20 includes a first solid electrolytelayer 21A, a second solid electrolyte layer 21B, a first alumina layer22A, a second alumina layer 22B, a third alumina layer 22C, a fourthalumina layer 22D, a fifth alumina layer 22E, a sixth alumina layer 22F,a diffusion-limited layer 23, a heater 24, a pump cell 25, a first pumpelectrode 25A, a second pump electrode 25B, a sensor cell 26, a firstsensor electrode 26A, a second sensor electrode 26B, a first atmosphericair introduction passage 27A, a second atmospheric air introductionpassage 27B, and an interior space 28.

Each of the solid electrolyte layers 21A and 21B is a layer formed ofzirconia or the like and has the oxygen ion conductive property. Thealumina layers 22A to 22F are layers formed of alumina, respectively.The diffusion-limited layer 23 is a porous layer, through which theexhaust gas can flow. In the sensor 20, the layers are laminated suchthat the sixth alumina layer 22F, the fifth alumina layer 22E, thefourth alumina layer 22D, the second solid electrolyte layer 21B, thediffusion-limited layer 23 and the third alumina layer 22C, the firstsolid electrolyte layer 21A, the second alumina layer 22B, and the firstalumina layer 22A are positioned in order from the lower side of FIG.22. The heater 24 is positioned between the fifth and sixth aluminalayers 22E and 22F.

The first atmospheric air introduction passage 27A is a space defined bythe first alumina layer 22A, the second alumina layer 22B, and the firstsolid electrolyte layer 21A, and a part of the first atmospheric airintroduction passage 27A opens to the atmosphere. The second atmosphericair introduction passage 27B is a space defined by the second solidelectrolyte layer 21B, the fourth alumina layer 22D, and the fifthalumina layer 22E, and a part of the second atmospheric air introductionpassage 27B opens to the atmosphere. The interior space 28 is a spacedefined by the first solid electrolyte layer 21A, the second solidelectrolyte layer 21B, the diffusion-limited layer 23, and the thirdalumina layer 22C, and a part of the interior space 28 communicates withthe outside of the sensor 20 via the diffusion-limited layer 23.

The first and second pump electrodes 25A and 25B are electrodes formedof material having low reducing performance (for example, an alloy ofgold and platinum), respectively. The first pump electrode 25A ispositioned on one of opposite surfaces of the second solid electrolytelayer 21B (that is, a surface of the second solid electrolyte layer 21Bwhich defines the interior space 28). The second pump electrode 25B ispositioned on the other surface of the second solid electrolyte layer21B (that is, a surface of the second solid electrolyte layer 21B whichdefines the second atmospheric air introduction passage 27B). The firstpump electrode 25A, the second pump electrode 25B, and the second solidelectrolyte layer 21B form the pump cell 25.

The exhaust gas discharged from the engine 50 flows into the interiorspace 28 through the diffusion-limited layer 23. The first pumpelectrode 25A exposes to the exhaust gas flowing into the interior space28.

The sensor 20 is configured to be able to apply the direct voltage fromthe pump cell voltage source 25C to the pump cell 25 (in particular, tothe second pump electrode 25B so as to procedure an electric potentialdifference with respect to the first pump electrode 25A). It should benoted that the first pump electrode 25A is a cathode side electrode, andthe second pump electrode 25B is an anode side electrode when the pumpcell voltage source 25C applies the direct voltage to the pump cell 25.

When the voltage is applied to the pump cell 25, and the oxygen in theinterior space 28 contacts the first pump electrode 25A, the oxygenbecomes the oxygen ion on the first pump electrode 25A and then, theoxygen ion moves toward the second pump electrode 25B through the secondsolid electrolyte layer 21B. At this time, the electric currentproportional to the amount of the oxygen ion, which has moved throughthe second solid electrolyte layer 21B, flows between the first andsecond pump electrodes 25A and 25B. Then, when the oxygen ion reachesthe second pump electrode 25B, the oxygen ion becomes the oxygen on thesecond pump electrode 25B and then, is discharged to the secondatmospheric air introduction passage 27B. Therefore, the pump cell 25can discharge the oxygen from the exhaust gas to the atmosphere by apumping function, thereby decreasing the oxygen concentration in theinterior space 28. An ability of the pumping function of the pump cell25 increases as the voltage applied to the pump cell 25 from the pumpcell voltage source 25C increases.

The first and second sensor electrodes 26A and 26B are electrodes formedof material having high reducing performance (for example, platinumgroup element such as platinum and rhodium or alloy of the platinumgroup element). The first sensor electrode 26A is positioned on one ofopposite surfaces of the first solid electrolyte layer 21A (that is, asurface of the first solid electrolyte layer 21A which defines theinterior space 28). The second sensor electrode 26B is positioned on theother surface of the first solid electrolyte layer 21A (that is, asurface of the first solid electrolyte layer 21A which defines the firstatmospheric air introduction passage 27A). The first sensor electrode26A, the second sensor electrode 26B, and the first solid electrolytelayer 21A form the sensor cell 26.

The first sensor electrode 26A exposes to the exhaust gas flowing intothe interior space 28 through the diffusion-limited layer 23.

The sensor 20 is configured to be able to apply the voltage from thesensor cell voltage source 26C to the sensor cell 26 (in particular, tothe second sensor electrode 26B so as to produce an electric potentialdifference with respect to the first sensor electrode 26A). The sensorcell voltage source 26C is configured to apply the direct voltage to thesensor cell 26. It should be noted that the first sensor electrode 26Ais a cathode side electrode, and the second sensor electrode 26B is ananode side electrode when the sensor cell voltage source 26C applies thedirect voltage to the sensor cell 26.

When the voltage is applied to the sensor cell 26, and the SOx in theinterior space 28 contacts the first sensor electrode 26A, the SOxdecomposes on the first sensor electrode 26A, the oxygen component ofthe SOx becomes the oxygen ion and then, the oxygen ion moves toward thesecond sensor electrode 26B through the first solid electrolyte layer21A. At this time, the electric current proportional to the amount ofthe oxygen ion, which has moved through the first solid electrolytelayer 21A, flows between the first and second sensor electrodes 26A and26B. When the oxygen ion reaches the second sensor electrode 26B, theoxygen ion becomes the oxygen on the second sensor electrode 26B andthen, is discharged to the atmospheric air introduction passage 27A.

The heater 24, the pump cell voltage source 25C, the sensor cell voltagesource 26C, the pump cell ammeter 25D, the sensor cell ammeter 26D, andthe sensor cell voltmeter 26E are electrically connected to the ECU 90.

The ECU 90 controls an activation of the heater 24 to maintain atemperature of the sensor cell 26 at the sensor activating temperature,at which the sensor 20 is activated.

In addition, the ECU 90 controls the voltage of the pump cell voltagesource 25C to apply the voltage set as described later to the pump cell25 from the pump cell voltage source 25C.

In addition, the ECU 90 controls the voltage of the sensor cell voltagesource 26C to apply the voltage set as described later to the sensorcell 26 from the sensor cell voltage source 26C.

The pump cell ammeter 25D detects a current Ipp flowing through acircuit including the pump cell 25 and outputs a signal representing thedetected current Ipp to the ECU 90. The ECU 90 acquires the current Ippon the basis of the signal. Hereinafter, the current Ipp will bereferred to as “the pump current Ipp”.

The sensor cell ammeter 26D detects a current Iss flowing through acircuit including the sensor cell 26 and outputs a signal representingthe detected current Iss to the ECU 90. The ECU 90 acquires the currentIss on the basis of the signal. Hereinafter, the current Iss will bereferred to as “the sensor current Iss”.

The sensor cell voltmeter 26E detects a voltage Vss applied to thesensor cell 26 and outputs a signal representing the detected voltageVss to the ECU 90. The ECU 90 acquires the voltage Vss on the basis ofthe signal. Hereinafter, the voltage Vss will be referred to as “thesensor voltage Vss”.

Summary of Operation of Second Embodiment Apparatus Acquisition ofExhaust SOx Concentration

Similar to the sensor 10, the inventors of this application have afollowing knowledge about the exhaust SOx concentration in the sensor20. The inventors increased the sensor voltage Vss from 0.4 V to 0.8 Vand then, decreases the sensor voltage Vss from 0.8 V to 0.2 V with thevoltage Vpp capable of reducing the oxygen concentration in the interiorspace 28 to zero (or generally zero) being applied to the pump cell 25.The inventors acquired the sensor current Iss as a base current Ibasewhen the sensor voltage Vss reached the oxygen decreasing voltage Vox_deafter the sensor voltage Vss started to decrease. In addition, theinventors acquired the sensor currents Iss(1) to Iss(m) after the sensorvoltage Vss reached the oxygen decreasing voltage Vox_de. The inventorsacquired an integration value S21 of differences dIss(n) (=Ibase−Iss(n))between the base current these and each of the sensor currents Iss(n)which are the sensor currents Iss(1) to Iss(m) acquired while the sensorvoltage Vss decreases after the sensor voltage Vss reaches the oxygendecreasing voltage Vox_de. In this case, the exhaust SOx concentrationincreases as the integration value S21 increases.

Accordingly, the second embodiment apparatus executes a constant voltagecontrol for controlling the sensor voltage Vss to maintain the sensorvoltage Vss at 0.4 V with the voltage Vpp capable of reducing the oxygenconcentration in the interior space 28 to zero (or generally zero) beingapplied to the pump cell 25 when the exhaust SOx concentration is notrequested to be acquired.

When the exhaust SOx concentration is requested to be acquired, and theengine operation is in the steady operation state or the idlingoperation state, the second embodiment apparatus executes theabove-described concentration acquisition voltage control.

The second embodiment apparatus acquires the sensor current Iss as thebase current Ibase when the sensor voltage Vss reaches the oxygendecreasing voltage Vox_de (in this embodiment, 0.6 V) while the secondembodiment apparatus executes the concentration acquisition voltagecontrol. Further, the second embodiment apparatus acquires the sensorcurrent Iss as the SOx concentration current Iss_sox(n) each time thesensor voltage Vss decreases by a predetermined value while the secondembodiment apparatus decreases the sensor voltage Vss from 0.6 V to thevoltage decreasing end voltage Vdown_end (in this embodiment, 0.2 V). Inaddition, the second embodiment apparatus stores the acquired SOxconcentration currents Iss_sox(n) in the RAM in association) with thesensor voltage Vss(n), at which the SOx concentration currentsIss_sox(n) are acquired.

Then, the second embodiment apparatus acquires the differences betweenthe base current Ibase and each of the SOx concentration currentsIss_sox(n) as the current differences dIss(n) (=Ibase−Iss_sox(n)). Thesecond embodiment apparatus acquires the integration value of thecurrent differences dIss(n) as the integration value S21 (=Σ(dIss(n))).

The second embodiment apparatus applies the acquired integration valueS21 to a look-up table Map21Csox(S21) to acquire the exhaust SOxconcentration Csox. The look-up table Map21Csox(S21) is preparedpreviously on the basis of experiments, etc. for determining arelationship between the integration value S21 and the exhaust SOxconcentration in the sensor 20. The exhaust SOx concentration Csoxacquired from the look-up table Map21Csox(S21) increases as theintegration value S21 increases. Hereinafter, the integration value S21will be referred to as “the current difference integration value S21”.

The second embodiment apparatus starts to execute the constant voltagecontrol for increasing the sensor voltage Vss from 0.2 V to 0.4 V andmaintaining the sensor voltage Vss at 0.4 V after the second embodimentapparatus stops executing the concentration acquisition voltage control.

The second embodiment apparatus acquires the exhaust SOx concentrationCsox, using the current difference integration value S21. The currentdifference integration value S21 is a value correlating with the exhaustSOx concentration. Therefore, the second embodiment apparatus canacquire the exhaust SOx concentration.

Further, (1) the base current Ibase and the SOx concentration currentIss_sox(n) used for acquiring the current difference integration valueS21 are currents subject to the oxidizing reaction of the sulfurcomponent derived from the SOx, (2) the current difference dIss used foracquiring the current difference integration value S21 is a value whichincludes no or almost no component of the sensor current Iss not subjectto the oxidizing reaction of the sulfur component, (3) the currentdifference integration value S21 is a value acquired using the SOxconcentration currents Iss_sox(n).

Therefore, a change of the current difference integration value S21 whenthe exhaust SOx concentration changes, is larger than a change of thedifference between the base current Ibase and the SOx concentrationcurrent Iss_sox when the exhaust SOx concentration changes in case thatthe sensor current not subject to the oxidizing reaction of the sulfurcomponent is used as the base current Ibase. Thus, the currentdifference integration value S21 represents the change of the exhaustSOx concentration definitely. The second embodiment apparatus acquiresthe exhaust SOx concentration Csox, using the current differenceintegration value S21. Thus, the second embodiment apparatus can acquirethe exhaust SOx concentration accurately.

The second embodiment apparatus applies the voltage Vpp capable ofdecreasing the oxygen concentration in the interior space 28 to zero (orgenerally zero) to the pump cell 25 when the second embodiment apparatusexecutes the preliminary voltage control and the concentrationacquisition voltage control. Thus, the oxygen concentration of theexhaust gas reaching the first sensor electrode 26A is generallyconstant even when the state of the engine operation changes while thesecond embodiment apparatus executes the preliminary voltage control andthe concentration acquisition voltage control. Therefore, the secondembodiment apparatus may be configured to execute the preliminaryvoltage control and the concentration acquisition voltage control andacquire the exhaust SOx concentration Csox even when the engineoperation is not in any of the steady operation state and the idlingoperation state after the exhaust SOx concentration is requested to beacquired.

Acquisition of Exhaust NOx Concentration

When the exhaust gas includes nitrogen oxide (hereinafter, will bereferred to as “NOx”), the NOx is reduced by the sensor cell 26 with thesensor voltage Vss being maintained at 0.4 V and decomposes to nitrogenand the oxygen. The oxygen produced by the NOx decomposing, becomes theoxygen ion at the sensor cell 26. The oxygen ion moves toward the secondsensor electrode 26B through the first solid electrolyte layer 21A.

Even when the voltage Vpp capable of reducing the oxygen concentrationin the interior space 28 to zero or generally zero, is applied to thepump cell 25, the NOx included in the exhaust gas is unlikely to bereduced at the pump cell 25 since the first and second pump electrodes25A and 25B forming the pump cell 25 are made of the material having thelow reduction property. In addition, when the voltage Vpp capable ofreducing the oxygen concentration in the interior space 28 to zero orgenerally zero, is applied to the pump cell 25, almost no oxygen isincluded in the exhaust gas reaching the sensor cell 26.

Therefore, when the voltage Vpp capable of reducing the oxygenconcentration in the interior space 28 to zero or generally zero, isapplied to the pump cell 25, and the sensor voltage Vss is maintained at0.4 V, the sensor current Iss output in proportion to the amount of theoxygen ion moving through the first solid electrolyte layer 21A, isproportional to a concentration of the NOx included in the exhaust gasjust discharged from the engine 50. Hereinafter, the concentration ofthe NOx will be referred to as “the NOx concentration”, and theconcentration of the NOx included in the exhaust gas just dischargedfrom the engine 50 will be referred to as “the exhaust NOxconcentration”. There is a relationship shown in FIG. 23 between thesensor current Iss and the exhaust NOx concentration. Therefore, theexhaust NOx concentration can be acquired by using the sensor currentIss.

Accordingly, the second embodiment apparatus executes a pump voltagecontrol for applying the voltage Vpp capable of reducing the oxygenconcentration in the interior space 28 to zero or generally zero to thepump cell 25 and the constant voltage control for controlling the sensorvoltage Vss to 0.4 V. The second embodiment apparatus acquires thesensor current Iss as a NOx concentration current Iss_nox while thesecond embodiment apparatus executes the pump voltage control and theconstant voltage control. Then, the second embodiment apparatus appliesthe NOx concentration current Iss_nox to a look-up tableMapCnox(Iss_nox), thereby acquiring the exhaust NOx concentration Cnox.The look-up table MapCnox(Iss_nox) is prepared previously on the basisof experiments, etc. for determining a relationship between the sensorcurrent Iss and the exhaust NOx concentration in the sensor 20. Theexhaust NOx concentration Cnox acquired from the look-up tableMapCnox(Iss_nox) increases as the NOx concentration current Iss_noxincreases.

Acquisition of Exhaust Oxygen Concentration

There is a relationship as shown in FIG. 3 between the voltage Vppapplied to the pump cell 25 from the pump cell voltage source 25C andthe pump current Ipp. Accordingly, the second embodiment apparatusexecutes the pump voltage control for applying the voltage Vpp capableof reducing the oxygen concentration in the interior space 28 to zero orgenerally zero to the pump cell 25. The second embodiment apparatusacquires the pump current Ipp as an oxygen concentration current Ipp_oxywhile the second embodiment apparatus executes the pump voltage control.Then, the second embodiment apparatus applies the oxygen concentrationcurrent Ipp_oxy to a look-up table MapCoxy(Ipp_oxy), thereby acquiringthe exhaust oxygen concentration Coxy. The look-up tableMapCoxy(Ipp_oxy) is prepared previously on the basis of experiments,etc. for determining a relationship between the pump current Ipp and theexhaust oxygen concentration in the sensor 20. The exhaust oxygenconcentration Coxy acquired from the look-up table MapCoxy(Ipp_oxy)increases as the oxygen concentration current Ipp_oxy increases.Hereinafter, the voltage Vpp applied to the pump cell 25 from the pumpcell voltage source 25C will be referred to as “the pump voltage Vpp”.

Thereby, the second embodiment apparatus can acquire the exhaust oxygenconcentration as well as the exhaust SOx concentration and the exhaustNOx concentration.

It should be noted that a relationship between the sensor voltage Vssand the sensor current Iss is the same as the relationship shown in FIG.3. Therefore, the second embodiment apparatus may be configured toacquire the sensor current Iss as the oxygen concentration currentIss_oxy while the second embodiment apparatus controls the sensorvoltage Vss to 0.4 V and the pump voltage Vpp to 0 V and apply theoxygen concentration current Iss_oxy to a look-up tableMapCoxy(Iss_oxy), thereby acquiring the exhaust oxygen concentrationCoxy. The exhaust oxygen concentration Coxy acquired from the look-uptable MapCoxy(Iss_oxy) increases as the oxygen concentration currentIss_oxy increases.

Concrete Operation of Second Embodiment Apparatus

Next, a concrete operation of the second embodiment apparatus will bedescribed. Similar to the first embodiment apparatus, the CPU of the ECU90 of the second embodiment apparatus is configured or programmed toexecute the routines shown in FIGS. 9 and 11 each time the predeterminedtime elapses.

Therefore, the second embodiment apparatus executes the routine shown inFIG. 10 at the step 960 in FIG. 9. In this case, the CPU of the secondembodiment apparatus acquires the current difference integration valueS21 at the step 1030 in FIG. 10 and applies the current differenceintegration value S21 to the look-up table Map21Csox(S21) to acquire theexhaust SOx concentration Csox.

Further, the CPU of the second embodiment apparatus executes processesof steps 2480 to 2490 shown in FIG. 24 in place of executing theprocesses of the steps 980 to 990 shown in FIG. 9.

When the value of the SOx concentration acquiring request flag Xsox is“0” at the time of executing the process of the step 905 in FIG. 9, theCPU of the second embodiment apparatus determines “No” at the step 905and then, executes the processes of the steps 2480 to 2490 in FIG. 24described below. Also, when the engine operation is not in any of thesteady operation state and the idling operation state at the time ofexecuting the process of the step 910 in FIG. 9, the CPU of the secondembodiment apparatus determines “No” at the step 910 and then, executesthe processes of the steps 2480 to 2490 in FIG. 24 described below.Then, the CPU of the second embodiment apparatus proceeds with theprocess to the step 995 in FIG. 9 to terminate this routine once.

Step 2480: The CPU of the second embodiment apparatus starts to executethe constant voltage control for controlling the sensor voltage Vss to0.4 V when the CPU has not executed the constant voltage control. On theother hand, the CPU of the second embodiment apparatus continues toexecute the constant voltage control when the CPU already executes theconstant voltage control.

Step 2485: The CPU of the second embodiment apparatus acquires the pumpcurrent Ipp as the oxygen concentration current Ipp_oxy and the sensorcurrent Iss as the NOx concentration current Iss_nox.

Step 2487: The CPU of the second embodiment apparatus applies the NOxconcentration current Iss_nox to the look-up table MapCnox(Iss_nox) toacquire the exhaust NOx concentration Cnox.

Step 2490: The CPU of the second embodiment apparatus applies the oxygenconcentration current Ipp_oxy to the look-up table MapCoxy(Ipp_oxy) toacquire the exhaust oxygen concentration Coxy.

It should be noted that the CPU of the second embodiment apparatuscontrols the pump cell voltage source 25C such that the pump voltage Vppcapable of reducing the oxygen concentration of the exhaust gas in theinterior space 28 to zero or generally zero, is applied to the pump cell25 while the second embodiment apparatus executes the routine shown inFIG. 9.

The second embodiment apparatus can acquire the exhaust SOxconcentration, the exhaust NOx concentration, and the exhaust oxygenconcentration by executing the routine shown in FIG. 9. In addition, thesecond embodiment apparatus can determine that the exhaust SOxconcentration is larger than the upper limit concentration Cth byexecuting the routine shown in FIG. 11.

Modified Example of Second Embodiment

Next, the SOx concentration acquiring apparatus of the internalcombustion engine according to a modified example of the secondembodiment will be described. Hereinafter, the SOx concentrationacquiring apparatus according to the modified example of the secondembodiment will be referred to as “the second modified apparatus”.

Summary of Operation of Second Modified Apparatus

Similar to the sensor 10, in the sensor 20, an integration value S22,which increases as the exhauast SOx concentration increases, can beacquired by acquiring the change rate of the sensor current Iss as thebase current change rate R while the sensor voltage Vss changes from thehigh-voltage current Ihigh to the low-voltage current Ilow, acquiringcurrents at the sensor voltages Vss(n), at which the SOx concentrationcurrents Iss_sox(n) are acquired, as the base currents Ibase(n), fromcurrents changing from the high-voltage current Ihigh at the basecurrent change rate R, acquiring differences between each of the basecurrents Ibase(n) and each of the SOx concentration currents Iss_sox(n)as the current differences dIss(n), and acquiring the integration valueS22 of the current differences dIss(n).

Accordingly, the second modified apparatus acquires the exhaust SOxconcentration Csox by executing processes similar to the processesexecuted by the first modified apparatus. The second modified apparatusexecutes the preliminary voltage control when the exhaust SOxconcentration is requested to be acquired, and the engine operation isin any of the steady operation state and the idling operation state. Thesecond modified apparatus acquires the low-voltage current Ilow whilethe second modified apparatus executes the preliminary voltage control.

The second modified apparatus acquires the sensor current Iss as the SOxconcentration current Iss_sox(n) each time the sensor voltage Vssdecreases by a predetermined value while the second modified apparatusdecreases the sensor voltage Vss from 0.6 V to 0.2 V after the secondmodified apparatus stops executing the preliminary voltage control. Inaddition, the second modified apparatus stores the acquired SOxconcentration currents Iss_sox(n) in the RAM in association with thesensor voltage Vss(n), at which the SOx concentration currentsIss_sox(n) are acquired.

Then, the second modified apparatus acquires an integration value ofdifferences between the reference current Iref and each of the SOxconcentration currents Iss_sox(n) as a first integration value S221(=Σ(Iref−Iss_sox(n))).

Further, the second modified apparatus acquires the sensor current Issas the high-voltage current Ihigh when the sensor voltage Vss reachesthe oxygen decreasing voltage Vox_de (in this example, 0.6 V) while thesecond modified apparatus executes the reoxidation voltage decreasingcontrol.

The second modified apparatus acquires the average change rate of thesensor current Iss changing from the high-voltage current Ihigh to thelow-voltage current Ilow as the base current change rate R. In addition,the second modified apparatus acquires the currents at the sensorvoltages Vss(n), at which the SOx concentration currents Iss_sox(n) areacquired, as the base currents Ibase(n) from the currents changing fromthe high-voltage current Ihigh at the base current change rate R.

The second modified apparatus acquires an integration value ofdifferences between the reference current Iref and each of the basecurrents Ibase(n) as a second integration value S222(=Σ(Iref−Iss_sox(n)))

The second modified apparatus subtracts the second integration valueS222 from the first integration value S221, thereby acquiring a currentdifference integration value S22(=S221−S222).

In other words, the second modified apparatus acquires the differencesbetween each of the base currents Ibase(n) and each of the SOxconcentration currents Iss_sox(n) as the current differences dIss(n) bythe above-described method. In addition, the second modified apparatusacquires the integration value of the current differences dIss(n) as thecurrent difference integration value S22.

The second modified apparatus applies the current difference integrationvalue S22 to a look-up table Map22Csox(S22) to acquire the exhaust SOxconcentration Csox. The look-up table Map22Csox(S22) is preparedpreviously on the basis of experiments, etc. for determining arelationship between the current difference integration value S22 andthe exhaust SOx concentration in the sensor 20. The exhaust SOxconcentration Csox acquired from the look-up table Map22Csox(S22)increases as the current difference integration value S22 increases.

The second modified apparatus acquires the exhaust SOx concentrationCsox, using the current difference integration value S22. The currentdifference integration value S22 is a value correlating with the SOxconcentration of the exhaust gas reaching the first sensor electrode26A. Therefore, the exhaust SOx concentration can be acquired.

Further, the base current change rate R is a value near the change rateof the sensor current Iss changing after the sensor voltage Vss reachesthe oxygen decreasing voltage Vox_de while the exhaust SOx concentrationis zero. Therefore, the differences between each of the base currentsIbase(n) and each of the SOx concentration currents Iss_sox(n) are valuein which components of the sensor current Iss not subject to theoxidizing reaction of the sulfur component are eliminated more orsubstantially since the currents acquired on the basis of the basecurrent change rate R is used as the base currents Ibase(n). Thus, thechange rate of the current difference integration value S22 is largerthan the change rate of the difference between the base current and theSOx concentration current Iss_sox acquired, using the current notsubject to the oxidizing reaction of the sulfur component as the basecurrent when the exhaust SOx concentration changes. Therefore, thecurrent difference integration value S22 represents the change of theexhaust SOx concentration definitely. Thus, the exhaust SOxconcentration can be acquired more accurately.

Concrete Operation of Second Modified Apparatus

Next, a concrete operation of the second modified apparatus will bedescribed. The CPU of the ECU 90 of the second modified apparatus isconfigured or programmed to execute the routines shown in FIGS. 15 and11 each time the predetermined time elapses.

When the CPU of the second modified apparatus executes the routine shownin FIG. 15, the CPU of the second modified apparatus executes theroutine shown in FIG. 17 at the step 1540 in FIG. 15 and executes theroutine shown in FIG. 18 at the step 1750 in FIG. 17. The CPU of thesecond modified apparatus acquires the current difference integrationvalue S22 at the step 1835 in FIG. 18. The CPU of the second modifiedapparatus applies the current difference integration value S22 to thelook-up table Map22Csox(S22) to acquire the exhaust SOx concentrationCsox at the step 1840 in FIG. 18.

Further, the CPU of the second modified apparatus executes the processesof the steps 2480 to 2490 in FIG. 24 in place of the processes of thesteps 1550 to 1570 in FIG. 15.

The CPU of the second modified apparatus also controls the pump cellvoltage source 25C such that the pump voltage Vpp capable of reducingthe oxygen concentration of the exhaust gas in the interior space 28 tozero or generally zero is applied to the pump cell 25 while the CPU ofthe second modified apparatus executes the routine shown in FIG. 15.

The CPU of the second modified apparatus can acquire the exhaust SOxconcentration Csox, the exhaust NOx concentration Cnox, and the exhaustoxygen concentration Coxy by executing the routine shown in FIG. 15.Further, the second modified apparatus can determine that the exhaustSOx concentration is larger than the upper limit concentration byexecuting the routine shown in FIG. 11.

It should be noted that the present invention is not limited to theaforementioned embodiment and various modifications can be employedwithin the scope of the present invention.

What is claimed is:
 1. A SOx concentration acquiring apparatus of aninternal combustion engine, comprising: a sensor cell formed by a solidelectrolyte layer, a first sensor electrode provided on one of oppositesurfaces of the solid electrolyte layer such that the first sensorelectrode exposes to an exhaust gas discharged from the internalcombustion engine, and a second sensor electrode provided on the othersurface of the solid electrolyte layer; and an electronic control unitfor controlling a sensor voltage which is a voltage applied to thesensor cell and acquiring a sensor current which is a current flowingthrough the sensor cell, wherein the electronic control unit isconfigured to: execute a decomposition voltage increasing control forincreasing the sensor voltage from a first voltage lower than an oxygenincreasing voltage to a second voltage equal to or higher than theoxygen increasing voltage, the oxygen increasing voltage being avoltage, at which an amount of oxygen component produced by SOxdecomposing to sulfur component and the oxygen component is larger thanthe amount of the oxygen component consumed by the sulfur componentbeing oxidized by the oxygen component to the SOx; execute a reoxidationvoltage decreasing control for decreasing the sensor voltage from thesecond voltage to a third voltage lower than an oxygen decreasingvoltage after the electronic control unit executes the decompositionvoltage increasing control, the oxygen decreasing voltage being avoltage, at which the amount of the oxygen component consumed by thesulfur component being oxidized by the oxygen component to the SOx islarger than the amount of the oxygen component produced by the SOxdecomposing to the sulfur component and the oxygen component; acquirethe sensor currents as SOx concentration currents, respectively afterthe sensor voltage reaches the oxygen decreasing voltage while theelectronic control unit executes the reoxidation voltage decreasingcontrol; acquire the sensor current as a base current when the sensorvoltage is equal to or lower than the oxygen decreasing voltage whilethe electronic control unit executes the reoxidation voltage decreasingcontrol; acquire an integration value of differences between the basecurrent and each of the SOx concentration currents; and acquire a SOxconcentration of the exhaust gas on the basis of the integration value.2. The SOx concentration acquiring apparatus as set forth in claim 1,wherein the electronic control unit is further configured to set, as thethird voltage, the sensor voltage, at which all the sulfur component isexpected to be reoxidized while the electronic control unit executes thereoxidation voltage decreasing control.
 3. The SOx concentrationacquiring apparatus as set forth in claim 1, wherein the electroniccontrol unit is further configured to: execute a constant voltagecontrol for controlling the sensor voltage to a voltage lower than theoxygen increasing voltage before the electronic control unit executesthe decomposition voltage increasing control after the electroniccontrol unit executes the reoxidation voltage decreasing control; andacquire an oxygen concentration of the exhaust gas on the basis of thesensor current acquired while the electronic control unit executes theconstant voltage control.
 4. The SOx concentration acquiring apparatusas set forth in claim 1, wherein the SOx concentration acquiringapparatus comprises the solid electrolyte layer as a first solidelectrolyte layer, the SOx concentration acquiring apparatus furthercomprises a pump cell formed by a second solid electrolyte layer, afirst pump electrode provided on one of opposite surfaces of the secondsolid electrolyte layer such that the first pump electrode exposes tothe exhaust gas, and a second pump electrode provided on the othersurface of the second solid electrolyte layer, and the electroniccontrol unit is further configured to: execute a pump voltage controlfor applying a voltage capable of decreasing an oxygen concentration ofthe exhaust gas to generally zero to the pump cell and a constantvoltage control for controlling the sensor voltage to a constant voltagelower than the oxygen increasing voltage; and acquire a NOxconcentration of the exhaust gas on the basis of the sensor currentacquired while the electronic control unit executes the pump voltagecontrol and the constant voltage control.
 5. The SOx concentrationacquiring apparatus as set forth in claim 4, wherein the electroniccontrol unit is further configured to acquire an oxygen concentration ofthe exhaust gas on the basis of a pump current which is a currentflowing through the pump cell while the electronic control unit executesthe pump voltage control.
 6. The SOx concentration acquiring apparatusas set forth in claim 1, wherein the SOx concentration acquiringapparatus further comprises a pump cell formed by the solid electrolytelayer, a first pump electrode provided on one of the opposite surfacesof the solid electrolyte layer such that the first pump electrodeexposes to the exhaust gas, and a second pump electrode provided on theother surface of the solid electrolyte layer, and the electronic controlunit is further configured to: execute a pump voltage control forapplying a voltage capable of decreasing an oxygen concentration of theexhaust gas to generally zero to the pump cell and a constant voltagecontrol for controlling the sensor voltage to a constant voltage lowerthan the oxygen increasing voltage; and acquire a NOx concentration ofthe exhaust gas on the basis of the sensor current acquired while theelectronic control unit executes the pump voltage control and theconstant voltage control.
 7. The SOx concentration acquiring apparatusas set forth in claim 6, wherein the electronic control unit is furtherconfigured to acquire an oxygen concentration of the exhaust gas on thebasis of a pump current which is a current flowing through the pump cellwhile the electronic control unit executes the pump voltage control. 8.A SOx concentration acquiring apparatus of an internal combustionengine, comprising: a sensor cell formed by a solid electrolyte layer, afirst sensor electrode provided on one of opposite surfaces of the solidelectrolyte layer such that the first sensor electrode exposes to anexhaust gas discharged from the internal combustion engine, and a secondsensor electrode provided on the other surface of the solid electrolytelayer; and an electronic control unit for controlling a sensor voltagewhich is a voltage applied to the sensor cell and acquiring a sensorcurrent which is a current flowing through the sensor cell, wherein theelectronic control unit is configured to: execute a decompositionvoltage increasing control for increasing the sensor voltage from afirst voltage lower than an oxygen increasing voltage to a secondvoltage equal to or higher than the oxygen increasing voltage, theoxygen increasing voltage being a voltage, at which an amount of oxygencomponent produced by SOx decomposing to sulfur component and the oxygencomponent is larger than the amount of the oxygen component consumed bythe sulfur component being oxidized by the oxygen component to the SOx;execute a reoxidation voltage decreasing control for decreasing thesensor voltage from the second voltage to a third voltage lower than anoxygen decreasing voltage after the electronic control unit executes thedecomposition voltage increasing control, the oxygen decreasing voltagebeing a voltage, at which the amount of the oxygen component consumed bythe sulfur component being oxidized by the oxygen component to the SOxis larger than the amount of the oxygen component produced by the SOxdecomposing to the sulfur component and the oxygen component; acquirethe sensor currents as SOx concentration currents, respectively afterthe sensor voltage reaches the oxygen decreasing voltage while theelectronic control unit executes the reoxidation voltage decreasingcontrol; acquire the sensor current as a high-voltage current when thesensor voltage decreases to a fourth voltage equal to or lower than theoxygen decreasing voltage; acquire the sensor current as a low-voltagecurrent when the sensor voltage decreases to a fifth voltage lower thanthe fourth voltage; acquire a change rate of the sensor current as asensor current change rate while the sensor current changes from thehigh-voltage current to the low-voltage current; acquire currents whichchange from the high-voltage current at the sensor current change rateand correspond to the sensor voltages, at which the electronic controlunit acquires the SOx concentration currents, as base currents,respectively; acquire an integration value of differences between eachof the base currents and each of the SOx concentration currents; andacquire a SOx concentration of the exhaust gas on the basis of theintegration value.
 9. The SOx concentration acquiring apparatus as setforth in claim 8, wherein the electronic control unit is furtherconfigured to acquire the sensor current as the high-voltage currentwhen the sensor voltage reaches the fourth voltage.
 10. The SOxconcentration acquiring apparatus as set forth in claim 8, wherein theelectronic control unit is further configured to set the oxygendecreasing voltage as the fourth voltage.
 11. The SOx concentrationacquiring apparatus as set forth in claim 8, wherein the electroniccontrol unit is further configured to: execute a preliminary voltageincreasing control for increasing the sensor voltage to a sixth voltagelower than the fourth voltage and the oxygen decreasing voltage; executea preliminary voltage decreasing control for decreasing the sensorvoltage from the sixth voltage to a voltage equal to or lower than thefifth voltage after the electronic control unit executes the preliminaryvoltage increasing control; and acquire the sensor current as thelow-voltage current when the sensor voltage reaches the fifth voltagewhile the electronic control unit executes the preliminary voltagedecreasing control.
 12. The SOx concentration acquiring apparatus as setforth in claim 8, wherein the electronic control unit is furtherconfigured to acquire the sensor current as the low-voltage current whenthe sensor voltage reaches the fifth voltage while the electroniccontrol unit executes the reoxidation voltage decreasing control. 13.The SOx concentration acquiring apparatus as set forth in claim 8,wherein the electronic control unit is further configured to set, as thethird voltage, the sensor voltage, at which all the sulfur component isexpected to be reoxidized while the electronic control unit executes thereoxidation voltage decreasing control.
 14. The SOx concentrationacquiring apparatus as set forth in claim 8, wherein the electroniccontrol unit is further configured to: execute a constant voltagecontrol for controlling the sensor voltage to a voltage lower than theoxygen increasing voltage before the electronic control unit executesthe decomposition voltage increasing control after the electroniccontrol unit executes the reoxidation voltage decreasing control; andacquire an oxygen concentration of the exhaust gas on the basis of thesensor current acquired while the electronic control unit executes theconstant voltage control.
 15. The SOx concentration acquiring apparatusas set forth in claim 8, wherein the SOx concentration acquiringapparatus comprises the solid electrolyte layer as a first solidelectrolyte layer, the SOx concentration acquiring apparatus furthercomprises a pump cell formed by a second solid electrolyte layer, afirst pump electrode provided on one of opposite surfaces of the secondsolid electrolyte layer such that the first pump electrode exposes tothe exhaust gas, and a second pump electrode provided on the othersurface of the second solid electrolyte layer, and the electroniccontrol unit is further configured to: execute a pump voltage controlfor applying a voltage capable of decreasing an oxygen concentration ofthe exhaust gas to generally zero to the pump cell and a constantvoltage control for controlling the sensor voltage to a constant voltagelower than the oxygen increasing voltage; and acquire a NOxconcentration of the exhaust gas on the basis of the sensor currentacquired while the electronic control unit executes the pump voltagecontrol and the constant voltage control.
 16. The SOx concentrationacquiring apparatus as set forth in claim 15, wherein the electroniccontrol unit is further configured to acquire an oxygen concentration ofthe exhaust gas on the basis of a pump current which is a currentflowing through the pump cell while the electronic control unit executesthe pump voltage control.
 17. The SOx concentration acquiring apparatusas set forth in claim 8, wherein the SOx concentration acquiringapparatus further comprises a pump cell formed by the solid electrolytelayer, a first pump electrode provided on one of the opposite surfacesof the solid electrolyte layer such that the first pump electrodeexposes to the exhaust gas, and a second pump electrode provided on theother surface of the solid electrolyte layer, and the electronic controlunit is further configured to: execute a pump voltage control forapplying a voltage capable of decreasing an oxygen concentration of theexhaust gas to generally zero to the pump cell and a constant voltagecontrol for controlling the sensor voltage to a constant voltage lowerthan the oxygen increasing voltage; and acquire a NOx concentration ofthe exhaust gas on the basis of the sensor current acquired while theelectronic control unit executes the pump voltage control and theconstant voltage control.
 18. The SOx concentration acquiring apparatusas set forth in claim 17, wherein the electronic control unit is furtherconfigured to acquire an oxygen concentration of the exhaust gas on thebasis of a pump current which is a current flowing through the pump cellwhile the electronic control unit executes the pump voltage control.